zorg,
is that 1800 fps a muzzle velocity, or velocity at a range having decayed from a higher muzzle velocity?
if it is from a higher muzzle velocity, how do you calculate the stability at the lower velocity.
I suspect there is more spin decay than some realize, but do not know how to calculate it.
Aberdeen proving ground did establish an amount of spin decay for a 308 win load, and from memory it was about 10%, at 1000 yds, but that could be wrong.
muzzle velocity is part of the stability formula, but I don't think anyone would use an mv of 1800 fps in anything but a pistol.
bruce.
It was a rough-and-ready assumption that spin rate of decay was almost negligible thus a a muzzle velocity.
Using 10% rotational velocity decay (at what distance or at what linear velocity as a % of muzzle velocity would be good to know), a 1:10 inches ratio would then be a 1:11 inches twist rate, correct?
Using 1:11 inches ratio and 1800 fps (muzzle but not really) velocity yields the following result using a model with the Miller twist rule, no polymer tip, 175 grain Nosler Partition, 1 atmosphere, 59°F, 50% humidity (reasonable representative South Texas Whitetail season conditions for my use), the progam spits out the following results.
"While theoretically stable, you're on the edge. A faster twist is suggested to ensure a suitable margin of safety."
If we use Wikipedia's value of 2595 fps for a 7mm-08 cartridge, 1800 fps is 69% of muzzle velocity, and using same atmospheric conditions and without introducing variable wind conditions, Fereral's ballistics app shows 1805 fps occurs at 380 yards from the muzzle. Of course, lower muzzle velocities will decay to 1800 fps at a shorter distance from the muzzle and simultaneously a higher % of muzzle velocity. This would bring into play the importance of the basis of 10% rotational velocity decay with respect to distance traveled from the muzzle and / or % of muzzle velocity where the 10% rotational velocity occurs, as well as the + or - bracketing of where the 10% rotational velocity decay occurs.
Of course all sorts of additional modeling requirements and refinements can be put into such models and cranked through, but again, what is the expected influence of such refinements on the actual rsults, and also within the precision of the model itself? Thee is a concept of Significant Figures that should be applied whenever building and using such models, plus the ability to actually repeatedly measure and validate the precision of such refinements.
My model framing is hunting use within "reasonable" - typical? - hunting target rnge applications with the 7mm 175 grain Nosler Partition projectile which has a published G1 BC = 0.387. If the discussion was framed around long range metal silhouette shooting with custom 7mm-08 rifles with barrel twist rates of say 1:8 inches or faster with projectiles such as the Federal 7mm 155 grain Edge TLR projectile the model precision and levels of appropriate refinement would be completely different.
So within the bound of what would be professionally described as my engineering judgement, the rough-and-ready modeling I performed for this example within the framework of use of the model and its results, my approach & results rmainvalid after adding a 10% generalized rotational velocity decay at an undefined distance from muzzle or % of muzzle velocity envelope of applicability of validity a generalized 10% is applicable at. Note the projectile minimum impact expansion velocity is a fixed value independent of muzzle velocity (and therefore distance from muzzle and % muzzle velocity) as well as atmospheric conditions.
If there is additional definition of the envelope where a rotational velocity decay of 10% is applicable, my engineering judgement is this is unlikely to significantly affect modeling predictions within the confines of the model and circumstances used to define the use of the model and its results unless a high sensitivity between rate of rotational velocity decay vs % of muzzle velocity (and by extension distance traveled from muzzle) is shown to radically change the generalized additional refinement basis of 10% rotational velocity decay introduced. (This is what is called a sensitivity analysis in my profession.)
I hope that's helpful not only from a perspective of adding a generalized refinement with undefined sensitivity, as well as directional effects of other refinements in general with respect to model use and framework of any model (as well as the concept of significant figures and by extension significnce of effects). But all of this is absoutely useless if pedantic adherance to the definitin of the trm "muzzle velocity" is insistently slavishly adhered to rather than conceptual use of extrapolating use of the model at any condition other than specifically what the assignned model terms are.
Take it for what itis, or reject it absolutely in the absence of a model based on an empirical data set with many many measurements taken mid-flight at 1800 fps for this specific projectile.
I never said my model extrapolations and results were in any way absolutely accurate to a defined number of significant figures. Instead what I alluded to erroneously thinking it went without explicit statement and demonstration, was my use of such model provides reasonable results in my engineering judgement within the context and framework of use of that model in the absence of me finding a model explicitly based on an empirical data set measured directly at the defined point of 1800 fps mid-flight.
I find the approach and results reasonable for my purposes and applicability of the general regime indicated by the rsultswithn two digits whch is the regime where changes to the regimes occur within the model, as well as a rational basis of how I extended the defined terms of the model I used by not being constrained by pedantic definition of one model input term. YMMV.
Edit: I guess I should add: is there is an element of risk this post will cause you to define yourself as my enemy? I honestly don't know of a better response and demonstration of the rationale of this response, plus rationale of any prior post I've made.
One central concept: if the model result indicates an unstable projectile, or stable projectile, does that mean that at any and every point in time (and distance) after thhe instant that projectile leaves the muzzle, does that really mean the projectile will be unstable, or stable, until forces of resistance of the fluid media the projectile is traveling through bring the projectile velocity to zero in all directions?
I posit if the model result says unstable, the answr is yes. If the model result says stable, the answer is no. Within the framework the model is used. Agree, or disagree?
Keep writing, keep us informed and look after your health. 
