Israeli Mauser 7.62 (.308) Twist rate and bullet weight

Discussion in 'Up To .375' started by Gert Odendaal, Jun 24, 2015.

  1. sestoppelman

    sestoppelman AH ENABLER AH Ambassador

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    Yes, familiar with the basic twist rates for bullets, but that has nothing to do with FB or BT. Bullets must be tried to determine performance.
     

  2. Gert Odendaal

    Gert Odendaal AH Elite

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    "some .300 Magnum barrels are rifled 1 turn in 14 inches. CHUCK CHAWS
    " This is interesting????
     

  3. ZG47

    ZG47 AH Enthusiast

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    It is worth remembering that boat-tail target bullets, with their short (very short for VLD bullets) shanks, do not always obdurate properly in standard rifling. Centrfire target-rifle barrels tend to be tighter than general purpose barrels because they are intended for use with boat-tail bullets only; and special larger diameter boat-tail target bullets are made to extend the accuracy life of worn target barrels.

    I have heard of shooters using .303 British projectiles in worn .30 cal barrels. Friends of mine use the Hornady SST projectile for .303 British (almost unobtainium and possibly developed for someone else to load 7.65 Argentine ammo) but it only achieves accuracy in their .303 Epps hunting rifles when propelled at 2,900 FPS average muzzle velocity. It is not an excessively long projectile, therefore insufficient obturation is the obvious issue in that instance.

    Projectile length is one factor. Shank length is another, both in terms of keeping the bullet straight as it enters the leade (conical progression to the rifling, after the parallel-sided throat) and in achieving proper obturation.

    If you want a simple demonstration of the influence of successful obturation on accuracy:

    Take a zeroed air rifle and a couple of standard diabolical pellets used to zero it. Deform the head of one pellet slightly and shoot it at a target. Then deform the skirt of the second pellet, shoot it at the same target and see how far it deviates from point of aim

    Gert, any 150 gr flat-base bullet with lead core should suffice for your shooting. A 170 gr flat-base flat-nose bullet might be the heaviest practical bullet and then ... possibly out to a maximum of 75 metres or so. By the way, if you want to do some positional target shooting with a light load, Lapua make a special .308 (.3075 actual diameter in all probability) target bullet in 100 gr???? I plan to get some myself for testing but it is one of many possible actions that I have been pondering for a while, i.e. I have been too damn lazy to get on with it!
     
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  4. Gert Odendaal

    Gert Odendaal AH Elite

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    ZG47, thank you for the info it is much appreciated. I really need to get the set-up working since it is no use to me to have a rifle that I can not shoot accurate with confidence. I rather then will sell this rifle and use the money to invest in my 8 x 68 S building project, I already have a buyer for the Israeli Mauser...I will give it one chance , buying flat based bullets in 150 gn weight , shoot them and see what accuracy I get...if there are good groups I will persist, if not I will sell this rifle ..I will keep you members up to date..(y)(y)(y)(y)
     

  5. ZG47

    ZG47 AH Enthusiast

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    Gert, all good. Not much point having all those conversations with highly competent shooters and gunsmiths over the years (and reading the sources they recommend) if you do not honour their courtesy by passing the info on to other people.
     

  6. Gert Odendaal

    Gert Odendaal AH Elite

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    It is true, I believe knowledge needs to be pass on , the learning curve never ends ...myself always try to learn as much as possible from my friend Johan Greyling, a brilliant gunsmith..it really is great to have a mentor of his caliber...I am truly fortunate...(y)(y)(y)
     

  7. Von Gruff

    Von Gruff AH ENABLER GOLD SUPPORTER AH Legend

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    Does Johan not have any insights into the possible problem Gert. Has he had a decent look at the barrel with a borescope to eliminate that end of possibilities for the inaccuracy? Have you run a lapp through to ensure there is not a discrepency in the bore or groove diameter.
     

  8. Gert Odendaal

    Gert Odendaal AH Elite

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    Garry, I always try to figure out a challenge myself since it is my own rifle . Eventually if I do not solve the problem I consult Johan Greyling. Working on a client`s rifle I always consult /ask Johan Greyling first.(y)(y)(y)
     

  9. Gert Odendaal

    Gert Odendaal AH Elite

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    Members, I am back, contemplating this challenge again..from the info you members provided me I located some bullets to buy if possible that may be use as a starting point to get a bullet that will be accurate in the 1:14 twist military stepped barrel ...here is some info , please provide your opinions to solve this conundrum..
    http://www.selwayarmory.com/barnes-...-frangible-flat-base-lead-free-box-of-50.html

    Another possibility??
    http://www.selwayarmory.com/barnes-...le-shock-x-flat-base-lead-free-box-of-50.html
     

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  10. Gert Odendaal

    Gert Odendaal AH Elite

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    Some comments from other sources on the net:
    ""Berger is a good resource to determine required twist. Look for bullet types that you plan to shoot, and see what the max wt Berger recommends. It is really the length of the bullet that requires more twist. So boat tails and low drag bullets need more twist for the same weight."

    "On a quick look, 155 grains seems to be about the limit for boat tails in a 14. You might get away with a bit more if the bullet is a flat base.
    Neil, The answer largely depends on the distance at which you plan to shoot the rifle. If you're shooting at 100 yards, you can probably do just fine with a bullet as heavy as 190 gr. and possibly more. As you go further out, the heavier bullet's marginal stability becomes more apparent in the form of decreased accuracy. For long range shooting, a 155 is the practical limit for that rate of twist, but up to 300 meters, you can likely shoot a much heavier bullet effectively.

    I realize this runs a bit counter to most conventional wisdom, but that's why I like to actually get out there and try things. I recently tested bullets up to 200 grains in a 1:13" twist .30-06 at moderate velocities and was pleasantly surprised by the results.

    You can see the targets and read the article here: http://riflemansjournal.blogspot.com/2010/06/ballistics-heavy-bullets-in-113-twist.html""""

    ""Re: 1/14 Twist .308

    Shooting 155 scennars with 30 inch 1/13 krieger barrel.
    Pushing 3000 fps,best group 5 1/2 at 1000 yards,love shooting this .308""

    '''I think that is a no go. Too slow to stabilize a 155, even when you are running at super fast velocities. I think I read about Mike Miller having a slow twist that would not stabilize 155's. I think it was 1:13" or 1:14".

    155's may be light, but you have to remember that they are also very long for their weight. It is the length that is more difficult to stabilize.

    Now if you wanted to shoot flat base bullets, you could get away with a 1:14". Not with BTHP's in 155 g:""""""


    ""The grand conflict in picking twist rates is the faster you spin the bullet the more it wobbles in flight, but the slower you spin it, the shorter the bullet has to be to stabilize adequately. Benchrest frequently favors light, flat base bullets because they tend to be shorter and that allows a slower twist with less wobble. Their drawback is at long range where the relatively lower ballistic coefficients make these bullets more vulnerable to wind."""

    "The G7 works better with flat base than the G1 because of the nose form being a better match. If you want a really good match to a flat base, though, the G6 drag curve is better."

    :""""
    The 1 in 14 will stabilize up to a 150 grain bullet (custom barrel).

    The 1 in 12 will stabilize up to a 168 grain bullet nicely.

    The 1 in 10 will stabilize up to a 220 grain bullet nicely."""

    ""http://www.shilen.com/calibersAndTwists.html

    .308
    - 8" for bullets heavier than 220gr.
    - 10" for bullets up to 220gr.
    - 12" for bullets up to 170gr.
    - 13"* Ratchet rifled 4 groove
    - 14"* for bullets up to 168gr.
    - 15"* for bullets up to 150gr.
    - 17"* for bullets up to 125 gr."""

    ""https://cuttingedgebullets.com/308-132gr-mtac-match-tactical""https://cuttingedgebullets.com/308-132gr-mtac-match-tactical

    A lot of info to process...(y)(y)(y)
     

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  11. Gert Odendaal

    Gert Odendaal AH Elite

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    More info to sort out:
    ""Possibly, machining set up costs keep 1-14 twist barrels common to Ruger Hawkeyes, Predator, and Target/Varmint .22-.250. Custom dictates 1-14 and most shooters being conservative are reluctant to change.""

    Chew on this one...this will keep you busy..as pointed out these are sources I got from the web..I now acknowledge the sources:

    ''Bullet spin rate is mind boggling .... the formula is: RPM= 12/twist rate ... times velocity (in fps) ... times 60. As an example: a .224" 55gr FMJ bullet fired from a 24" barreled 223 Rem rifle @ 3240 fps with a T/R of 1:12 is: 12/12=1; 3240X1=3240; 60X3240=194,400 RPM. This example just happens to be the norm for 223 Rem bolt action rifles and will keep a 55gr FMJ stable to 300~350 yards, after which the bullet becomes unstable. Another example: a 62gr FMJ fired at 3000 fps with a 20" 1:9 twist barrel. 12/9=1.333; 3000X1.333=4000; 4000X60=240,000 RPM. This example just happens to be the standard for typical AR-15s and will keep a bullet stable to 400~500 yards. Finally, let's try a 1:7.5 twist and a 70 gr bulletfired at 3000 fps: 288,000 RPM, which will maintain stability to 1000 yards and is the preferred twist rate for long range shooters.

    Because your rifle's twist rate is "fixed", the only ways to maintain longer downrange stability is to 1), use bullets with a higher BC or 2) Increase muzzle velocity. This gets complicated because you now have another concern ... chamber pressure. Heavier bullets are also longer and typically have higher BCs. This means shorter range accuracy will not be as good (larger spiral so the bullet takes longer to go to sleep) and because you can't drive heavier bullets faster without exceeding chamber pressure limits, they will not maintain long rangestability either.

    One successful way to maintain better long range accuracy without forfeiting closer range accuracy is to use plastic tip bullets (ie Hornady V-Max or Nosler Ballistic tips). Why? Plastic is very light compared to the metal bullet so even though they are a bit longer than conventional bullets, their "effective" length is really just the length of the metal body. This keeps the initial spiral smaller (better close range accuracy) and because these types of bullets have a better BC than a typical FMJ, they will maintain better downrange stability where you can expect them to travel farther before the spin rate decays and causes them to become unstable. This will increase the stable range by about 100 yards.

    So what happens when you use a lighter bullet ... ie a 45 gr in a 223 Rem? For one, you can increase velocity without exceeding max chamber pressure. Lighter bullets are also shorter, however because they are driven faster, the combined effect from centrifugal force is about the same as a longer bullet so close range accuracy is about the same as a 55gr. Even though lighter bullets have a poorer BC, the extra velocity will spin them faster and help maintain stability to about the same distance as a 55gr bullet. So in essence, there's really no accuracy or distance advantage with lighter bullets, however at closer distances, they do smack the target with a bit more authority because of more retained velocity.
     

  12. Gert Odendaal

    Gert Odendaal AH Elite

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    Here is another great piece of logic:
    Now for your 22-250 ... these rifles typically have a slow 1:14 twist rate because the bullets are driven much faster than a 223 Rem. Example: 22-250 with a 55gr FMJ in a 1:14 twist barrelbeing driven at 3680 fps will develop about 189,250 RPM, versus 194,400 RPM for a 223 Rem with a 1:12 twist, making stability range about the same (300~350 yards). Note the RPMs with the above 62gr bullet in a fast twist AR-15 (240,000 RPM). If you go to a 62gr bullet in a 22-250, velocity drops to about 3400 fps and RPMs drops under 175,000. This is not fast enough to stabilize the bullet and will result in poor accuracy. 22-250s really shine with lighter bullets ... 40-45 gr. As an example, a 40gr factory load is rated at 4150 fps so it will develop 213,400 RPM ... plenty of spin to keep it stable past 400 yards. A 1:10 twist in the RAR allows you to use heavier bullets (up to 62gr) but because the barrel is shorter, it will NOT develop as much velocity as a 24~26" barrel.
     

  13. Gert Odendaal

    Gert Odendaal AH Elite

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    "When I had a 1-14, my bullet of choice was the .224 52gr. Sierra HPBT Match King. It appeared to have a tough jacket and I could shoot them out smoking hot."
    It seems long flat based bullets at high velocities will get accurate groupings near and far out on targets????
     

  14. Gert Odendaal

    Gert Odendaal AH Elite

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    Members , here is the punch line why a 1:14 twist rate is used as a varminter barrel/rifle.
    ""Getting back to the point of all this - why does the predator rifle have a 1-14 twist????---

    The answer is that is intended to shoot short but pointy bullets at relatively high velocities at practical varmint ranges of up to 400 yards but deliver pin point accuracy at 100 or even 50 yards. Very little drop and very short time of flight."""

    Yes, Hornady has some notes about the longer bullets. It says the 53 grain BT VMax will not work in 1:14 twists or slower.


    But they imply that the 75 grain BTHP will work. Odd. I have a few and will give them a try when I get my gun.

    What advantages are their to a 1:14 twist rate?
    rentsmill, Good questions and I'll try to address them. A little history first. Way back in the 1930's, varmint hunting became very popular. It was a way to develop good shooting skills and reduce the population of pests and varmints. As shooters became more proficient, the demand for longer range varmint shooting increased to a point where there were many "wildcat" cartridges ... some remain popular, most have died out. One of the hottest 22 cal varmint cartridges was the 220 Swift ... a real barn burner but also a real barrel burner. The 22-250 was initially a wildcat that was developed in the 1930's from a necked down 250 Savage case. It was finally standardized by Remington in 1965 and added to the list of SAAMI cartridges. When wildcatters experimented with different twist rates, they soon discovered .... any thing faster than 1:14 would shorten the life of the barrel ... the faster the twist, the shorter the life. For this reason, (until recently), all 22-250 gun manufacturers shipped their rifles with a 1:14 twist rate. If you wanted a faster twist barrel, you had to replace the factory barrel.

    The 22-250's slow 1:14 twist rate would not stabilize bullets weighing over 55 gr, in fact worked better with even lighter bullets. In recent years, there has been a demand for heavier bullets with higher BCs that would extend the range of a 22-250 from 300 yards to as much as 1000 yards so the only solution was to increase the twist rate so heavier bullets would stabilize. These demands came from long range shooters, not from varmint hunters.

    Each bullet (no matter what caliber) has an optimum spin rate that will keep it stable in flight for X yards. Because .224" bullets are very small in diameter, they don't produce as much gyro effect as larger diameter bullets so they have to be spun faster to maintain downrange stability. As an example: a 62 gr FMJ requires a spin rate of 220,000 RPM to maintain stability to 500 yards (or more). Anything less will cause the bullet's spin rate to decay to a point where it becomes unstable at a lesser distance ... start to wobble, and eventually tumble. It doesn't matter what gun or cartridge the bullet is fired from ... it's just the bullet's spin rate that counts and is different for each type bullet, bullet weight, and length. As you know, spin rate is a product of the rifle's twist rate and muzzle velocity. If you use the above bullet spin rate of 220,000 and "reverse engineer", you will find a 22-250 with a MV of 3500 fps needs a twist rate of 1:11.5 to achieve about 220,000 RPM. Heavier bullets increase chamber pressure so there is a limit on how fast a bullet can be driven without damaging the rifle. So ... the heavier the bullet, the slower the MV will be. The only way to make up for the loss of velocity is to increase the barrel's twist rate. Herein lies the problem with 22-250s and many other cartridges.

    With a normal 1:14 twist rate, 22-250s are known "barrel eaters" ... typically lasting 5000 rounds, whereas 50,000 rounds is quite common with other cartridges. As the twist rate increases, barrel life will diminish. Several factors come into play. The throats are the first thing to go. They will erode and cause excessive bullet damage, which in turn makes accuracy go south. You can compensate for eroded throats by increasing the OAL of the cartridge (seating the bullet out farther). This will extend barrel life by 500 rounds or more, however; as the throat erodes even more combined with bore wear, after about 5000 rounds, the barrel will need to be replaced. With heavier bullets and a faster twist rate, throat erosion will be about the same but because of a combination of heavier bullets and more resistance from a faster twist rate, the bore itself will wear much faster.

    Modern metallurgy has helped extend barrel life but not by huge amounts .... maybe 1000 rounds at most. So to answer your last question ... "Why not just make the RAR in a 1:8 twist rate and be done with it?" ... the answer is quite simple .... gun manufactures don't want to get a bad rap for selling something that will wear out sooner than it should and can't afford to replace barrels on customer returned rifles.

    Quote:
    Would some bullets explode from turning too fast?

    Yes, most bullet manufacturers provide the specifications for their bullets, which have an optimum and a maximum velocity rating. This assumes the bullets will be fired from standard twist rate barrels and can be downgraded proportionally by faster than normal twist rates. What actually happens is ... the centrifugal force generated by a combination of the rifle's twist rate and velocity can spin the bullet so fast that the jacket will shed. All bullets rated for super high velocities will have a much thicker jacket. Those bullets designed for varmints will literally explode on contact and will do much the same from excessive centrifugal forces.

    What advantages are their to a 1:14 twist rate?

    The primary advantage is barrel life. Now let's get practical. The 22-250 with a 1:14 twist is one of the best varmint rifles you can buy. It has a solid range of at least 300 yards and is very capable of pasting a coyote at that distance with one shot. I know this for a fact because I have done it. For smaller critters like woodchucks or prairie dogs, likely you won't even be able to see them at distances beyond 300 yards and even if you do, it better be a very calm day or wind drift will blow your bullet way off course. I think I went into detail in another post about the "spiral effect" that affect all bullets. When bullets are spun at an excessive RPM, it takes a lot longer distance for the spiral to dissipate and continue on a true course. You will find ... a 22-250 with a 1:14 twist will put a 55gr bullet to sleep at about 60 yards, whereas with a faster twist and heavier bullets, the "sleep distance" could extend well past 100 yards. So ... the advantage to a 1:14 twist is it will be more accurate at closer distances than a barrel with a faster twist. Again on the practical side ... it is very common for a prairie dog or woodchuck to pop his head out of a hole at 50 yards. With a fast twist, that little quarter sized target will likely be missed, whereas with a slow 1:14 ... instant hair, teeth, and eye ball vapor.

    Just my opinion but I think it is really true ... heavy bullets with faster twist rates are nothing more than a fad created by long distance shooters where they need bullet stability as far as 1000 yards. If you plan to shoot at these distances, then a faster twist with heavier bullets may be just the ticket. However if you are a varmint shooter or rarely shoot much past 300 yards, a standard 1:14 twist rate will serve you well and last much longer in the process. 300 yards is a loooong ways. Even with a high magnification scope, a prairie dog looks the same size as an ant and will be covered by the scope's crosshairs. There is no one twist rate that is perfect and no one bullet weight either .... so pick the one you will most likely use the most and forget about all the "what ifs" that clutter reasonable thinking.


    I think this is the baseline I need to work from to get an accurate load for the 1"14 RIFLE twis Israeli Mauser I have,,.....(y)(y)(y):LOL::LOL:
     
    Last edited: Oct 14, 2018
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  15. Gert Odendaal

    Gert Odendaal AH Elite

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    Another excellent piece of info:Ruger Forum content
    Lets kick the twist rate up to 1:9 for a 308 Win, which is intended for 165gr bullets at 2700 fps. If you shoot a 150gr bullet @ 2820 fps with this twist rate, centrifugal force will cause the bullet to be "over spun" ... some call it "over stabilized". Instead of the yaw spiral dissipating at 75 yards, it now takes about 125 yards. This means the bullet will still be in a fairly large spiral at 100 yards, which will show up as larger groups on paper. After 125 yards, the spiral will dissipate and allow the bullet to maintain true flight to at least 750 yards. With a 165grbullet and a 1:9 twist rate, because the bullet is longer and is being spun faster, it will take about 150 yards for the yaw spiral to dissipate, however it will maintain true flight for at least 1000 yards. So in conclusion, if you want long range accuracy out to 1000 yards, a faster spin rate with a heavier bullet should be used. If you want a more practical shooting range of 75~600 yards, a lighter bullet with a slower twist rate will work much better.

    Fast twist rate barrels wear much faster than slower twist rate barrel and higher velocity accelerates wear even more. As an example, let's say you had a rifle with a standard twist rate that had a barrel life expectancy of 5000 rounds with the intended ammo. By increasing the twist rate by just one inch (ie 1:10 versus 1:9), barrel life may be cut in half to 2500 rounds. No big deal for a hunting rifle ... it would take a lifetime to wear out a barrel. However, if you have a rifle with a shorter barrel life ... ie 243 Win, 22-250, 220 Swift, you could easily wear out a barrel with a faster twist rate in just a few years ... way less time if you shoot a lot. When I was young, I bought a SAKO Finbar chambered in 264 Win Mag for an antelope hunt. This is about the flattest shooting high power cartridge you can buy, however when I found out it had a normal barrel life expectancy of just 500 rounds, I sold it right after the hunt. Why such a short barrel life? A 1:9 twist coupled with a MV of 3030 fps with a 140gr bullet and a very steep shoulder angle makes 264 Win Mags about the worst barrel eaters you can find.
    [​IMG]
     
    Last edited: Oct 14, 2018

  16. Gert Odendaal

    Gert Odendaal AH Elite

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    Another one":Ruger Forum:

    As for the .30's. I would guess that the optimum twist for the common 155 grain target bullet fired from a .308 Win having a 30 inch barrel with maximum velocity hopes would be somewhat slower than 1-12. One good reason for this is higher velocities. The folks that get into this refer fast twists as "choking" bullet velocities.

    One reason why 1-10 twists are common for almost all .30's is that they can handle bullets from 110 to up to 200 grains or more with extremely good accuracy. Visiting a bench rest match where .30's were being used would reveal 1-13, 1-14 twists shooting stubby bullets of less than 150 grains. The 125 gr. Berger flat base is a good example. Practical experience with the stubby 125 gr. Speer TNT out of a 1-10 provides more than enough accuracy except for bench rest completion where terms such as "shoots in the tens" are used. Reduced course competition using the stubby 125 TNT varmint bullet at 100 yards or even 200 yards from a 1-10 that in the standard course is fired with 168's and 175's can produce winning scores. Many 100 yard reduced course matches have been won with 168's and at one time I used 168's for everything. This gets into the actual situation of how good can you hold not some minute and rather academic determination of accuracy.

    Bullet wobble - ballistic yaw and "yaw of repose"

    As the bullet flies out of the muzzle of the rifle it is usually in a slightly misaligned state, that is the axis of the bullet does not coincide with the flight path; this is caused by a huge blast of hot gasses that exit the muzzle at the same time as the bullet and natural vibrations of the rifle barrel. This dampens out at a certain distance, usually short of 150 yards. The angle of the deflection is very slight usually in the order of 1 degree (Sierra manual page 1060, ed. V). Oscillation, coning, or yaw occurs but has no affect on the path of the bullet other than the point of the bullet describing a very small and diminishing circle as it flies out. This is also called precession. Longer more pointy bullets exhibit more of the phenomena than short stubby ones do. Don't show up at a 100 yard bench rest match with a .300 Win Mag loaded with 208 Amax bullets and don't expect good results during a windy day 1000 yard shoot using your 1.5 inch .30 caliber wild cat loaded with 125 grain Bergers or the much lower priced 125 gr. TNT's. (shoots in the 20's @ 100). The initial yaw of the bullet is caused by forces such as powdergas blasts, barrel vibrations and possibly strong side winds not centrifugal force and gyroscopic stability is the only factor to remove this yaw or coning and depending on the length of the bullet usually does this within 150 yards of less. Sierra Manual page 1016-1017, ed. V.

    The Sierra manual describes this as "a small angular misalignment" and I have problems in understanding how a 168 gr. 30 bullet having a length of about 1.125 inches will result in a 2.5 inch spiral near the muzzle. Hunters initially getting their 1-10 twist .30-06's sighted in at 25 yards would never get the 1 inch or so groups that they usually obtain if this were true.

    Another effect is the "yaw of repose" this lasts as long as the bullet is in flight. This is caused by a combination of factors including aero dynamic forces (wind) directed at the front of the bullet which has a very slight tendency to be diverted by torque (spinning forces acting between the bullets center of mass and the center of pressure that is forward of the latter) so the axis of the bullet is very slightly diverted from the path. This can be noticed by the need to crank on more windage from moving from 300 to 600 yards (left as most rifles usually have right twists). Page 1063 Sierra manual, ed. V.

    For me, speaking from my experience, are 1-9 twists for .22's and 1-10 for .30's because they can shoot a great variety bullets with more than acceptable accuracy, that in some cases are one hole groups with 53 .22's, 125 30's and ability to shoot long skinny bullets like 75 .22's and 208, 210 .30's if needed. A sub 1 inch or better group at 100 yards using a 208 grain out of a .300 Win mag is possible.

    I believe the relative powder capacity of a cartridge combined with the established pressures for that cartridge and bore diameter are the primary factors determining barrel life. Compare the barrel life of" .308 Win vs. .300 Win Mag, .223 with .22-.250, .260 Rem with .264 Win Mag and so on. The fast barrel eaters use more powder and have higher pressures, usually over 61 thousand psi. Possibly a faster twist might have a shorter barrel life as higher pressures occur to cause more bullet spinning but to what degree. How much barrel life will a 1-12 .300 Win mag have over a 1-10. I have never seen any documented empirical data regarding this. A commonly heard item is that to using less twist avoids "choking" bullets allowing faster velocities. Another item of note and having controversy is neck length - will a .243 with its short stubby neck cause more barrel wear than the 6mm Rem with its more esthetic appearing shape having a longer neck; both being loaded to the same pressures. Will double base powders containing about 8% nitroglycerine eat out barrels faster than extruded single base powders containing only nitrocellulose. Match and high volume varmint shooters can go through a barrel in 3 years or less, commonly high scoring match shooters replace their .308 Win barrels after 2 years or about 3500 - 4000 rounds; a commonly heard complaint is that their 600 yard scores have fallen off. 1-10 and 1-12 .308s shooting 168's and 175's are common and the Hi Power course is 200, 300, and 600 yards. Faster twists than 1-10 are used for 220 - 240 gr. .30 caliber long boat tail match bullets.

    7.5/20 = .375 turn, .375*360 degrees = 135 degrees , 90 degrees + 45 degrees = 135, 90 degrees would be 1/4 turn and the extra 35 degrees would be 1/8 turn, 3/8 turn or midway between 1/4 and 1/2, 3/8 = .375, mark off the tape in 45 degree increments, .25 inch diameter rod has 3.14*.25 = .785 inch circumference, .785/8 = .098 or roughly .1 increments. No argument from me about asking the manufacturer about twist but what if you got different answers? As we all know specifications sometimes change.
     

  17. Gert Odendaal

    Gert Odendaal AH Elite

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    Acknowledge the Ruger forum where I copy these informative content from>
    BassMan, Bullet yaw is a great topic because there are many theories on why it hap
    pens ... all of which have validity, none of which are solely responsible. Breaking it down, there are four "causes" for bullet yaw ... the gun itself, the bullet, the cartridge, and the environment.

    Starting with the gun ... anything that causes the bullet to be damaged from the time the cartridge is loaded in the magazine until the bullet exits the muzzle will make the bullet something less than perfectly balanced. Any bullet that leaves the muzzle without being perfectly balanced will create yaw. This would include the gun's feeding system where a bullet may get skinned from a feed ramp that isn't shaped properly, or the bullet is forced into the chamber at an angle where it may get distorted. It could also be related to recoil where a cartridge in the magazine gets thrust forward hard enough where the nose of the bullet strikes the front of the magazine. Bullet distortion from feeding is more common with 22 LRs or other straight wall cartridges ... not as common with bottle neck cartridges but it does happen, especially with semi-autos.

    The first obstacle the bullet sees is the leade ... commonly called a throat. The leade's job is to throttle down the chamber from case mouth diameter to bore diameter ... typically about .020" and works much like a forcing cone. When the bullet (being thrust by very high pressure) strikes the leade, it ALWAYS creates some bullet damage. The severity of the damage will depend on two main factors ... bullet runout (cartridge not being perfectly straight, not the gun's fault) or the condition of the throat. All rifle throats will erode but some, depending on the cartridge, erode at a much faster rate. As you noted, the amount of powder and driving pressure are the main contributing factors. The shoulder angle of a bottle neck cartridge acts much like an optical lens where the pressure is "focused" on the base of the bullet. Cartridges with a sharp shoulder angle (when the body of the case is a much larger diameter than the diameter of the bullet), will focus more pressure on the throat and less on the base of the bullet. This will accelerate throat erosion much faster than a cartridge with a lesser shoulder angle. In most high power rifles, throat erosion will happen long before the bore wears out and here's why. All smokeless gun powder burns at temperatures between 5000 and 6000 deg F. This would be plenty hot enough to melt steel, however the saving grace is .. this extreme temperature is only applied for a fraction of a second. Larger charges of powder take longer to burn as do slower burn rate powders used in magnum cartridges. This will apply extreme heat for a longer period of time, acting much like a plasma cutter and in time flame cuts (erodes) the leade. Unfortunately, throats don't erode in a symmetrical pattern, rather they tend to erode leaving pits, which causes excessive damage to the bullet and makes it yaw downrange. Throat erosion is the main contributing factor for barrel life, not so much bore wear.

    Rifle bores are never perfect, however the more perfect they are, the less bullets will get damaged. The biggest contributing factor is the uniform depth of the rifling (lands). Lands engrave bullets so if they are something less than perfect, one land may cut deeper than the others, making the bullet exit the muzzle "out of balance", which in turn causes the bullet to yaw. Striation marks (machine marks) in the bore literally file off a tiny amount of the jacket, also making the bullet slightly out of round and causing yaw. This is the very reason why two identical make and model rifles may shoot different sized groups with the same ammo.

    Two other gun related issues ... barrels suffer from harmonics, which are vibrations (much like a tuning fork) in the barrel that cause the muzzle to move in a circular motion. If each bulletleft the muzzle at exactly the same velocity, it would be predictable where the muzzle was in the same exact arc position. In reality, this does not happen ... just a few fps will change barrelharmonics and release the bullet in a slightly different direction. If barrel vibrations (harmonics) are excessive, not only will the bullet exit in a slightly different direction, it will also cause the bullet to yaw slightly. The last significant gun issue is the barrel's crown. The purpose of a crown is to apply equal exit pressure on the entire circumference of the bullet. If the crown is just slightly imperfect, muzzle blast will push the base of the bullet in the direction where exit pressure was the highest, which in turn causes the bullet to yaw ... sometimes radically. The lighter the bullet, the more influence the crown will have.

    Bullets ... not all bullets are created equally. Match grade bullets tend to be way more perfect ... weight, diameter, and most importantly, they don't have internal air bubbles or voids that affects balance. When bullets are spun at very high RPMs (200,000 RPM or higher is quite common), the slightest imperfection will cause the bullet to spin in an erratic manner, which causes yaw. Downrange accuracy is directly related to yaw. As yaw increases due to bullet damage, groups will open up. Match grade bullets are the best, hunting grade bulletsare pretty good, "bulk grade" bullets are the worst. Usually you can determine the quality of the bullet by just weighing a sample batch. You will find ... match grade bullets will be at the exact advertised weight and will not vary enough to be detected on a normal reloading scale. Field grade (hunting) bullets also tend to be pretty uniform but may vary a few tenths of a grain or may vary slightly in length or diameter. It is not unusual to see bulk grade bullets vary several grains ... and most of them will not be at the advertised weight. So ... you could say inferior quality bullets come "pre-damaged".

    Bullets tend to be the most accurate when they are matched to the rifle's twist rate at standard factory velocities. Matching bullets really has more to do with length but because weight and length are typically proportional, we mostly think in terms of weight. One major exception is Barnes bronze bullets that are much longer than their conventional bullet counterparts (bronze bullets are lighter than lead core jacketed bullets). Shorter bullets create a smaller "yaw spiral" near the muzzle and the spiral dissipates much faster (shorter shooting distance) than longer bullets. It's not just the length, rather a ratio of diameter to length. In other words, a larger diameter bullet that is the same length as a smaller diameter bullet will dissipate the yaw spiral much faster than the smaller diameter bullet. One of the best long range cartridges is the 6.5x55 Swede, however it is also one of the worst for short range accuracy. That's because it takes a considerable distance for the long skinny bullet to dissipate the yaw spiral ... but once that happens, the bullets will maintain stability for a very long distance. Short fat bullets, such as a 150gr 30 cal, will totally dissipate the yaw spiral in a short distance, assuming a slow twist rate barrel. 22 LR bullets are perhaps the best example ... short, light weight, very slow twist rate (1:16) and a slow MV (1200 fps), which all contribute to minimal centrifugal force, making the small yaw spiral dissipate in just a few yards.

    Any bullet fired from any gun will create a yaw spiral. The diameter of the yaw spiral is a combination of the above mentioned causes, but the major contributing factor is the centrifugal force influenced by the rifles's twist rate coupled with muzzle velocity. In other words, the RPM of the bullet where higher RPMs create more centrifugal force. If it were not for imperfections in bullets, the effects from centrifugal force would not be near as dramatic ... just like a slightly unbalanced tire on you car is more noticeable at highway speeds than in town. Bullet RPMs are affected more by the twist rate than velocity. Using a 223 Rem as an example: a bullet fired at 3000 fps in a 1:12 twist rate barrel will develop 180,000 RPM. That same bullet fired at 3200 fps will develop 192,000 RPM. If the same bullet was fired in a 1:10 twist rate, the RPMs would be 216,000 @ 3000 fps or 230,400 RPM @ 3200 fps. As you can see, a slight difference in twist rate will have a much larger influence on RPM than velocity.

    When a bullet is matched to the rifle's twist rate at factory velocities, the yaw spiral is typically about the same diameter as the length of the bullet soon after it leaves the muzzle, however in a mismatched barrel with a faster twist rate, it could easily double. The larger the yaw spiral, the farther the bullet has to travel before it dissipates. All bullets will continue to yaw slightly as they travel downrange. This is always because of the damage the bullet incurred in the rifle and/or the quality of the bullet to start with. All bullets lose velocity as they travel downrange due to air friction and all bullet spin rates will decay downrange for the same reason. At some point downrange, the bullet's spin rate will no longer support the amount of gyro effect needed to keep it stabilized. At that point, yaw will take over and cause the bullet to wobble until it finally tumbles. With hunting rifles, this is never an issue because bullets will maintain stability long past the practical range of the rifle. With long range target rifles shooting at 1000 yards, loosing stability is a problem because most bullets won't travel that far without destabilizing. Here's where a faster twist rate comes into play ... with a higher RPM spin rate, the bullet will travel much farther before its spin rate decays enough to cause instability. The bullet's ballistic coefficient (BC) is a very important factor for long range shooting. The higher the BC, the better the bullet will cope with air resistance in both velocity and spin rate. Larger diameter bullets create a lot more gyro effect and typically have a much higher BC, thus they will maintain stability for a very long distance. .224 diameter bullets are a "worst case" example because they typically have very poor BCs and because of the small diameter, they don't generate as much gyro effect. This come with mixed results ... at distances out to a couple hundred yards, .224 FMJ bullets can be exceptionally accurate, however; for extreme long range accuracy, the bullet requires a much faster spin rate plus it must have a higher BC. A typical 55gr FMJ will have a BC of .240 whereas a good match grade 75gr .224" bullet will have a BC of .435 ... almost half the air resistance of the FMJ. Of course the 75gr bulletmust be spun much faster so it will travel 1000 yards before losing stability versus a 55gr FMJ where it becomes unstable as soon as 300 yards in a 1:12 twist barrel.

    As mentioned above, runout in any cartridge will cause bullet damage when the bullet strikes the throat just a fraction of a degree off center. Other cartridge related issues have more to do with shoulder angles. A radical shoulder angle does not mean the gun will be inaccurate or create excessive yaw but it does mean the throat will likely erode faster than normal, which will create bullet damage and yaw.

    The last effect on bullet yaw has to do with the environment ... mainly air density and wind. Any change in temperature, altitude, barometric pressure, or humidity will affect air density, which in itself does NOT cause bullets to yaw. Denser air will cause the yaw spiral to dissipate faster at closer distances, however at longer shooting distances, it has the opposite affect. This is because denser air slows the bullet's velocity and decays spin rate more. So ... at longer distances, natural bullet yaw from bullet damage will cause the bullet to yaw more and become unstable. Wind has a different affect. Because of the gyro effect, bullets want to travel in a straight path. If a side wind blows the bullet off course, it defies the gyro effect and will cause the bullet to yaw. The bullet's BC has a dramatic affect on wind drift and wind yaw. Higher BCs resist head winds and side winds better than lower BC bullets. As such, higher BC bullets are not blown off course (wind drift) as much as lower BC bullets so there is also less influence on wind yaw.

    Fun stuff ... and it might help explain a few quirks with rifles.
     
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  18. Gert Odendaal

    Gert Odendaal AH Elite

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    Bullets I am looking at to purchase for the Israeli Mauser stepped military barrel 24" bolt rifle :
    Hornady InterLock Bullets 30 Caliber (308 Diameter) 150 Grain Spire Point with Cannelure

    Barnes Bullets .30 Caliber (.308 Diameter) 110 Gr. Tipped TSX Flat Base-
    Another excellent review:
    https://www.pewpewtactical.com/best-308-ammo/

     
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  19. sestoppelman

    sestoppelman AH ENABLER AH Ambassador

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    I still have a hard time wrapping my noodle around the twist in your rifle. I have never heard of such a twist in a military barrel when 10, 11 or 12 would be the norm.
     
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  20. Gert Odendaal

    Gert Odendaal AH Elite

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    Sest, I did research this phenomenon..of a 1:14 twist rate in my 7.62 /.308 rifle barrel. This happened when Israel got Mauser rifles from I am standing to be corrected , Poland , these barrels were substituted and all German marks were removed...all types of barrels with different rifling twists were fit to the actions ...irony that Israel used there enemies the Germans production rifles to strengthen their army... your view on the information regarding twist rates and types of bullets I sourced from the internet??
    Flat-based bullets 150 gn at high velocities out to ghong shooting distances of 400 meters using this twist rate of 1:14 ???
     

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