question on the physics of obturation

sgt_zim

sgt_zim

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It's pretty common for lead bullets intended to be fired faster than 1700 or 1800 fps to have a gas check to prevent obturation of the base.

Then I look at the Partitions and A Frames on my reloading bench and wonder why they both have exposed lead at the base, and why isn't obturation an issue for them. Is it because they're enclosed by gilding metal on the load bearing surface with a bit of coverage on the base? That's it, that's the question.
 
Obturation is when the bullet swells up to groove diameter and fills the bore completely. Some bullets have a hollow base to facilitate this.
Gas checks really don’t prevent this they just keep all the hot expansion gasses behind the bullet instead of going around it.
With jacketed bullets it’s not an issue if gasses slip by the bullet because the bullet won’t leave lead in the barrel.
Jacketed bullets with exposed bases can’t really obturate because the jacket is too stout.
A FMJBT has exposed lead at the base, if the bullet obturated the boat tail would all be squished upwards till the base of the bullet was groove diameter, looking like a flat based bullet.
I imagine that any deformation of the exposed lead on the bases of jacketed bullets is very minimal, since it is so well contained by the gliding metal.
 
The most modern bullets designed by RWS, their Evolution, feature a "ballistic calotte" which I assume is for the purpose of obturation. Base of the bullet is concave, the opposite of a boat tail. They shoot very, very precisely in my rifles.
I expect the Nosler Partition does actually obturate to some extent. Not sure how you'd measure, since the bullet base can only expand to bore diameter and not more. Partitons I've recovered after firing often have impressions left by kernels of powder in the lead core of the base. If the powder is pressing hard enough to do that, I'd expect its pushing hard enough to obturate too.
 
@sgt_zim
building a 470 Capstick and given that most barrels were 0.475 groove and most bullets 0.474 (or even less), looked at the barnes tsx (yield strength ~40kpsi per barnes) bullet expansion vs chamber pressure.

1774532877453.jpeg
 
Jefferry404 said:
@sgt_zim
building a 470 Capstick and given that most barrels were 0.475 groove and most bullets 0.474 (or even less), looked at the barnes tsx (yield strength ~40kpsi per barnes) bullet expansion vs chamber pressure.

View attachment 755264
Click to expand...
I can see the pressure and distance measurements, but how in the hell can you measure throat/bullet diameter?????
 
sgt_zim said:
Then I look at the Partitions and A Frames on my reloading bench and wonder why they both have exposed lead at the base, and why isn't obturation an issue for them. Is it because they're enclosed by gilding metal on the load bearing surface with a bit of coverage on the base? That's it, that's the question.
Click to expand...
Can you provide some pictures?
 
throat is per the Manson reamer print, bullet obduration is calculated as the bullet compresses axially, it expands radially to conserve volume via Poisson's Ratio.
 
Jefferry404 said:
throat is per the Manson reamer print, bullet obduration is calculated as the bullet compresses axially, it expands radially to conserve volume via Poisson's Ratio.
Click to expand...
So it's calculated and not measured? I can't imagine how it would be measured (hence my question).
 
The lands can and will displace some bullet metal to fill the grooves. Of course it can only help for so far.
A fun fact is that most 9mm handguns have a .356 or .357 groove diameter, but nearly all 9mm bullets are .355.
 
sgt_zim said:
It's pretty common for lead bullets intended to be fired faster than 1700 or 1800 fps to have a gas check to prevent obturation of the base.

Then I look at the Partitions and A Frames on my reloading bench and wonder why they both have exposed lead at the base, and why isn't obturation an issue for them. Is it because they're enclosed by gilding metal on the load bearing surface with a bit of coverage on the base? That's it, that's the question.
Click to expand...
You are actually asking about the ‘gas check cup’ used on some small arm projectiles i.e., NOT the ‘gas check’ formerly used on rifled artillery projectiles.

A gas check is a copper or copper alloy disk, introduced in the latter half of the 19th century, to prevent propellant gas ‘blow by’ with artillery shells i.e., a contemporary update of the wooden disc sabot used with round shot in smooth bore cannons. The gas check was superseded by copper or copper alloy metal drive bands. N.B. The Luftwaffe changed to sintered iron bands on their 88mm flak projectiles during WW2 and barrel wear was reduced.

The copper gas check cup was a joint development by leading target shooter Dr Walter Hudson, a New York physician; and John Barlow, the founder of Ideal. Hudson was a key figure in the development of an accurate service bullet for the Krag rifle. An accurate bullet was eventually created, in collaboration with ‘UMC’ Thomas BUT barrel life with service ammunition was approximately 1,400 rounds. The gas check cup was developed during Hudson’s efforts, in conjunction with Barlow, to develop a cast bullet practice load for the Krag. It serves, as noted by other respondents, to protect the base of a cast lead bullet from melting.

Shortening of the term ‘gas check cup’ to ‘gas check’ has caused a great deal of unnecessary confusion amongst shooters. Have a good weekend.

P.S. Some forum members may wish to read Walter Hudson - Krag Specialist by Jim Foral, Gun Digest 62nd Ed, 2008.
 
Last edited:
sgt_zim said:
Partition on the left, A Frame on the right
Click to expand...
Make the casing, pour the lead.
Choose proper hard lead alloy for each section.
A frame, make bonding in front.
(probably all patented)

Why?
Simplification and economization of production process.

I was reading John Noslers "Going Ballistic", his autobiography book.
When he started he was making the bullets first by lathe machine for jacket, then added the lead.
 
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