Advanced rifle twist questions


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FNFiveSeven
April 2, 2003, 03:19 AM
Allright, here goes. I have a .223 AR w/ 16 inch bbl 1/9 twist and HS Precision in .308 w/ a 24 inch bbl 1/12 twist. I've been told that the ideal bullet weights for these barrels are 62 grain (M855) and 168 grains, respectively. But wait, not so fast!

As temperature goes down, and air density increases, the relative stability of the bullet traveling through the colder, "thicker" medium will decrease (for any given fixed twist rate and velocity). So the first question is, at how low of a temperature can I still expect these rifles to shoot their "ideal" rounds accurately?

Question 2: Has anyone ever documented a noticeable decrease in velocity between two barrels shooting the same round where the only difference is the twist rate of the barrel? I would expect the tighter twisting barrels to dump more of the powder charge's energy into rotational energy of the bullet, at the cost of translational energy... resulting in decreased velocity???

Question 3 (Thanks for sticking with me): Isn't all this talk about how different twists with different weight bullets affect the "yawing" performance of the bullet in flesh just a bunch of BS? I mean, think about it: a 1/9 twist is required to stabilize a 62 grain projectile at 3000 fps in *air*... and realize that flesh is thousands of times more dense than air on even the coldest day. So how could any .223 bullet weight, at any twist rate (yes, even a 55 grain in a 1/7 twist) possibly NOT be destabilized upon entering a solid/liquid target?

Thanks in advance for shedding light on any of these questions... these are some issues I haven't been able to resolve for some time and have been driving me nuts.

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JohnKSa
April 3, 2003, 12:25 AM
1. They're overspun enough that minor factors like temperature changes won't cause them to destabilize unless you start pushing the limits of what your gun can handle in terms of bullet length. I suppose you could keep going up in bullet weight until you found a bullet that was stable at say 100 degrees but unstable at freezing. It's not likely that you'll get into this kind of trouble with standard twists and reasonable bullet weights for those twists.

2. Spinning a bullet doesn't require a lot of energy. I suspect that minor manufacturing variations in barrels would have much more effect on velocity than twist rate.

3. Destabilization in a fluid medium is pretty independent of twist rate.

Art Eatman
April 3, 2003, 12:47 AM
I gotta go along with John. These sorts of variables might matter to somebody trying to set a new world's record in benchrest competition, but that's about it.

For instance, in my '06 I get reasonably tight groups with 110-grain, and very good groups with 150-, 165- and 180-grain bullets; inside one MOA, commonly.

My Ruger 77 MK II in .223 shoots equally well with 50-grain through blunt-nosed 70-grain bullets; about 1/2 MOA for 3-shot groups.

I've sighted rifles in during summertime, and seen no noticeable difference in cold weather on a re-check.

:), Art

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