ponchh
Member
Spot on with the bullet, powder and barrel length combo.hdwhit said:
Twist vs Barrel Length
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Kingcreek
Member
Twist. But the real reason I'm posting is to point out that "old school" is 1:12 in the AR. That came before 1:9 1:8 and 1:7.
Archie
Member
Perhaps I need to clarify. Rifling twist is most important for accuracy. This presumes the barrel is properly made, fits the bullet correctly, isn't corroded, off center, bent and so on. However, this is NOT to say faster is always better. Twist depends on the length and bore diameter of the bullet.
Bullets spun too slow tend to yaw and tumble in flight. Bullets spun too fast tend to show up any imbalance and wobble extensively. In my experience and all my research, a fair amount of 'over spin' is better than a tiny bit of 'under spin'. This suggests twist rate is a range, rather than a specific and 'single point' value. This is borne out by most rifles shooting more than one bullet weight fairly well.
Someone mentioned use of a .30 caliber rifle with 1 in 14 twist. U. S. Armed Forces rifles in .30 caliber - starting with .30 Army/.30-40 Krag, then .30-03/.30-06 and finally 7.62mm NATO - have all used 1 in 10 twist barrels and nearly all commercial rifles have done so as well. The .30 Army round used a 220 FMJ bullet, as did the .30-03. The 1 in 10 twist stabilized those long bullets well. When the .30-06 was ordained with a 150 (more or less) grain bullet, the manufacturing equipment was never changed; no reason to do so (in the minds of the decision makers). Currently, .30 caliber bullets, suitable for use in the .30-06 include 130 grain bullets. On a large scale, they cause no problem, even though the formula - and the practice - indicates a much slower twist acceptable.
Longer barrels allow pressure to act on the bullet, accelerating it until it leaves the barrel. However, this works only as long as the pressure in the bore is a certain percentage greater than the atmospheric pressure 'outside' the bore. And of course, a six foot long barrel would be awkward in most cases. (Wouldn't comfortably fit in the truck of my car, for that matter.) An excellent work on this subject is Firearms Pressure Factors, by Dr. Lloyd Brownell (Wolfe Press, not sure if still in print).
Another factor to remember is all rifles are different. I have yet to find any two rifles having identical 'best loads'. However, as demonstrated by factory ammunition and military 'issue' ammunition, one load can perform between 'adequate' and 'very good' in a number of rifles so chambered. And some rifles just 'don't like' certain brands of bullets, or certain bullet constructions, or certain powders.
Bullets spun too slow tend to yaw and tumble in flight. Bullets spun too fast tend to show up any imbalance and wobble extensively. In my experience and all my research, a fair amount of 'over spin' is better than a tiny bit of 'under spin'. This suggests twist rate is a range, rather than a specific and 'single point' value. This is borne out by most rifles shooting more than one bullet weight fairly well.
Someone mentioned use of a .30 caliber rifle with 1 in 14 twist. U. S. Armed Forces rifles in .30 caliber - starting with .30 Army/.30-40 Krag, then .30-03/.30-06 and finally 7.62mm NATO - have all used 1 in 10 twist barrels and nearly all commercial rifles have done so as well. The .30 Army round used a 220 FMJ bullet, as did the .30-03. The 1 in 10 twist stabilized those long bullets well. When the .30-06 was ordained with a 150 (more or less) grain bullet, the manufacturing equipment was never changed; no reason to do so (in the minds of the decision makers). Currently, .30 caliber bullets, suitable for use in the .30-06 include 130 grain bullets. On a large scale, they cause no problem, even though the formula - and the practice - indicates a much slower twist acceptable.
Longer barrels allow pressure to act on the bullet, accelerating it until it leaves the barrel. However, this works only as long as the pressure in the bore is a certain percentage greater than the atmospheric pressure 'outside' the bore. And of course, a six foot long barrel would be awkward in most cases. (Wouldn't comfortably fit in the truck of my car, for that matter.) An excellent work on this subject is Firearms Pressure Factors, by Dr. Lloyd Brownell (Wolfe Press, not sure if still in print).
Another factor to remember is all rifles are different. I have yet to find any two rifles having identical 'best loads'. However, as demonstrated by factory ammunition and military 'issue' ammunition, one load can perform between 'adequate' and 'very good' in a number of rifles so chambered. And some rifles just 'don't like' certain brands of bullets, or certain bullet constructions, or certain powders.
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