Posted: 5/3/2010 1:34:04 PM EDT
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on barrel rate of twist and what grain of bullet works best for .223/5.56.
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Barrel twist refers to how long it takes the rifling to make one complete turn, so 1:7 would be 1 full revolution in 7 inches. 1 in 9 is one turn in 9 inches.
Generally, the faster the twist (1:7 is faster than 1:9), the heavier the bullet the barrel will accommodate, though this is an indirect relationship. How well a bullet stabilizes in a barrel is actually a function of how long it is; it just happens that bullet length and weight in grains are almost always directly related, so the longer the bullet, the heavier, the faster twist rate required to stabilize. With a 1:9 twist, mid to upper 60's in grains is usually the limit for weight before the bullet becomes unstable. 1:7 twists can take the heavier 70+ grain bullets fine, generally speaking. There comes a point, however, at which the bullet becomes so heavy (and thus long) that it will not fit into a standard magazine when properly seated in the casing, nor will it allow the bolt of the AR to go fully into battery. Such "match-grade" bullets generally stipulate that the gun be "throated," which just means that it can accommodate them fully and safely. It is a one way street. 55gr is pretty standard for 1:9 twist barrels, but it will work just fine in a 1:7. A 75 gr bullet, on the other hand, which may function just fine and be plenty accurate out of a 1:7 barrel, is probably not going to be adequately stabilized in a 1:9 (slower) twist barrel. Stabilization and velocity at the muzzle is also a function of barrel length. All else held equal, the 5.56 round will attain a higher velocity, much closer to what it is intended to achieve, out of a 20" barrel than a shorter one. Some would argue more stable at the muzzle, too, as a result. In any event, it will stay stable for longer with the higher velocity, and be more accurate. This said, YMMV. It may be the case that you have a 1:9 barrel and you pick up some 70-something grain bullets and they do just fine in it; there is a certain amount of unique performance characteristics with each rifle, and the only way to get a feel for this is trial and error. I know about .000001% of what some of the big shots over in the ammunition forums know about this sort of thing (Molon, Zhukov, etc. come to mind). Edit: I realize you probably already know >95% of that stuff... this is a gun forum, after all, and you've been here longer than I have. I probably took your question WAYYY too literally and i dont mean to patronize; I'm nearly braindead from studying. Just pretend I didn't.. 
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I have a 1/8 twist BCM SS barrel that throws EVERYTHING 55 grain and up like a laser, fwiw. Out to what range? Case in point. Sometimes it just depends. I agree with that.....69 gr SMKs shine the most out of it out to about 300 yrds....at least that's as far as I've shot them. |
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TONS of good info in the ammo oracle. I refer to it often...
Arfcom Ammo Oracle It says this about twist rates... What bullet weight can I shoot in my (1/9, 1/8, 1/7, etc.) rifle?
This is undoubtedly one of the questions that is asked the most. The answer depends on many things, but here are some rough guidelines for those that don't care about the details: 1-in-14" 55gr or less 1-in-12" 35gr up to 55-60gr 1-in-9" 45gr-75gr, possibly up to 77 if you're lucky 1-in-8", 1-in-7" 45gr - 80+gr To find out for sure if your bullet will stabilize a given bullet which is on the fringe, you'll have to try it out in YOUR rifle. Some people's 1/9 can't stabilize 75gr bullets, while others can get away with 77gr bullets in theirs. Technically, it isn't about the WEIGHT, but the LENGTH of the bullet, as well as air density and velocity. A simplified method for determining the minimum rifling twist rate for a given length of bullet is given by the Greenhill formula. A Google search will show many sources for those interested in a more in-depth discussion. If you have the length of your bullet, and just want a look-up table to see what twist rate you need, refer to this page. If you really enjoy the in-depth mathematical treatment of the subject, check out these pages. I prefer 1 in 7 for versatility, but it really depends on what you are trying to accomplish w/ the rifle. |