Yep, and that says it all. I suspect the poor accuracy from many AR-15's using 55gr FMJ ammo stems from the quality of the ammo. Bulk produced, especially lots rejected by the military, is not likely to produce the accuracy that high quality ammo would produce in a given rifle.
Shot a friend's AR yesterday - he was clueless about twist rates, but I informed him last night
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From the top.
1:7 for the purple guy. The carbine's 1:9 and does it really matter with a 5.5" barrel?
I've been happily blasting away with 55 grain but according to the chart I can go a bit heavier.
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There is some accurate information in this thread, along with a whole lot of myth.
1) When the M855 62 gr round was developed it was well understood that 1:9 twist was the optimum twist rate for its length. However 1:7 was adopted due to a desire for the M16A2 to be able to fire the much longer M856 tracer round, with enough stability to penetrate a steel pot at 500m.
2) 1:12 twist is ideal for the M193 55 gr Type B bullet that was originally designed by Stoner and Sierra for the M16. Early 1;14 twist rifles had issue in extreme cold weather testing where the bullet was only marginally stable.
Remington however, used its own shorter type A bullet in production ammo, in part to use its own bullet and in part to resolve the stability issue before the government changed the rifling twist.
That's also what triggered the whole penetration, velocity, pressure problem as that shorter tangent ogive Type A bullet had a lower BC and bled a lot more velocity than the longer, secant ogive higher BC bullet it had bene designed for.
3) The problem with the average 55 gr FMJ in a 1: twist barrel is that FMJs are generally not as consistent. Their center of gravity is usually not all that precisely aligned with their center of form. Consequently, when they leave the muzzle they transition from the muzzle to free flight, they also transition from rotation around the center of form to rotation around the center of gravity, and that causes some initial wobble which causes yaw and precession, which increases dispersion.
At 3200 fps, a 1:12 twist is imparting 192,000 rpm to the bullet. However in a 1:7 twist barrel, that increases to 329,142 rpm. that much higher rotational velocity has a similarly large increase on the presession of the bullet as it yaws.
In other words, while a more precisely made FMJ like the Hornady 55 gr FMJBT will shoot better than a less precisely made 55 gr FMJ, the same 55 gr bullet at the same velocity will shoot better in a 1:12 twist barrel than in a 1:7 twist barrel. Excess rotation is never a good thing.
4) The 1:9 twist has become the standard commercial twist for very good reason. It does less harm to the accuracy of cheap 55 gr FMJs and its fast enough to stabilize bullets up to around 69 grains (depending on length and velocity).
5) From a service rifle competition perspective, I preferred 1:8 twist. It would adequately stabilize any bullet that could be loaded to magazine length, without excess spin that would reduce its accuracy potential.
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In short, if you're going to:
- shoot 55 gr FMJ range ammo, get 1:12;
- shoot 62 gr FMJ, or a combination of 55 and 62 gr, get 1:9;
- shoot magazine length match loads in weights up to around 75 grains, get 1:8; and
- if you are going to single load longer, heavier rounds in a bolt gun, then sure, 1:7 makes sense.
- if you're shooting a short barrel rifle or pistol with commensurately lower velocities and also shoot longer bullets heavier than 62 grains, then - and only then - 1:7 might make sense.
1) When the M855 62 gr round was developed it was well understood that 1:9 twist was the optimum twist rate for its length. However 1:7 was adopted due to a desire for the M16A2 to be able to fire the much longer M856 tracer round, with enough stability to penetrate a steel pot at 500m.
2) 1:12 twist is ideal for the M193 55 gr Type B bullet that was originally designed by Stoner and Sierra for the M16. Early 1;14 twist rifles had issue in extreme cold weather testing where the bullet was only marginally stable.
Remington however, used its own shorter type A bullet in production ammo, in part to use its own bullet and in part to resolve the stability issue before the government changed the rifling twist.
That's also what triggered the whole penetration, velocity, pressure problem as that shorter tangent ogive Type A bullet had a lower BC and bled a lot more velocity than the longer, secant ogive higher BC bullet it had bene designed for.
3) The problem with the average 55 gr FMJ in a 1: twist barrel is that FMJs are generally not as consistent. Their center of gravity is usually not all that precisely aligned with their center of form. Consequently, when they leave the muzzle they transition from the muzzle to free flight, they also transition from rotation around the center of form to rotation around the center of gravity, and that causes some initial wobble which causes yaw and precession, which increases dispersion.
At 3200 fps, a 1:12 twist is imparting 192,000 rpm to the bullet. However in a 1:7 twist barrel, that increases to 329,142 rpm. that much higher rotational velocity has a similarly large increase on the presession of the bullet as it yaws.
In other words, while a more precisely made FMJ like the Hornady 55 gr FMJBT will shoot better than a less precisely made 55 gr FMJ, the same 55 gr bullet at the same velocity will shoot better in a 1:12 twist barrel than in a 1:7 twist barrel. Excess rotation is never a good thing.
4) The 1:9 twist has become the standard commercial twist for very good reason. It does less harm to the accuracy of cheap 55 gr FMJs and its fast enough to stabilize bullets up to around 69 grains (depending on length and velocity).
5) From a service rifle competition perspective, I preferred 1:8 twist. It would adequately stabilize any bullet that could be loaded to magazine length, without excess spin that would reduce its accuracy potential.
-----
In short, if you're going to:
- shoot 55 gr FMJ range ammo, get 1:12;
- shoot 62 gr FMJ, or a combination of 55 and 62 gr, get 1:9;
- shoot magazine length match loads in weights up to around 75 grains, get 1:8; and
- if you are going to single load longer, heavier rounds in a bolt gun, then sure, 1:7 makes sense.
- if you're shooting a short barrel rifle or pistol with commensurately lower velocities and also shoot longer bullets heavier than 62 grains, then - and only then - 1:7 might make sense.
Good thread!
OP
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I do not personally own an AR but have to think because of the very light bullet weights (much lighter than many other rifle rounds) the bullets are inherently less stabile, hence the difference the twist rate matters. I suppose that some AR's can handle heavier or lighter bullets better than others but the results might also be affected by barrel length, distance, ammo velocity, how the rifles are sighted in and the individual shooters. There are some shooters who can squeeze every drop of accuracy out of even a less than perfect set up while others can barely hit the black consistency even with perfect set ups, twist rates and super accurate rifle/scope combos.
Twist rates? Most people don't even have a clue as to their chosen round's ballistics. I have heard things like, "my 300mag shoots flat out to 500 yards". Start adding things like barrel length, bullet weight, twist rate and the height of the scopes center line over the bores to the equation and 90% of the shooting public won't have a clue. Most of them only know what caliber of rifle they have and a few of them are not even sure of that. I am always amazed to find out what some haven't found out and don't really care to know.
drayks
Member
The ORIGINAL twist rate for military M16 was 1/14. In Arctic cold weather (dense air) testing they found out bullets wouldn't stabilize, hence VERY poor accuracy. Went to 1/12 immediately for 20" barrel and 55 grain FMJ used in Vietnam era. As poster BB57 states excellently, every lead core military FMJ "improvement" demanded an IDEAL twist rate. Bullets went up by approximately 7 grains each time i.e. 55/62/69/75 as AR twist rates went from 1/12--1/9--1/8-- and 1/7 as proven by millions of AR target shooters . The stories about light (45 grain) bullets disappearing in a puff of gray smoke fired in a 1/7 twist barrel halfway to a 100 yard target are true. Be mindful though that ALL COPPER bullets break the progression by being longer per unit of weight. The twist rate chart is an IDEAL, so if your ammo shoots to your satisfaction, have at it.
This guy has the correct history....
Years ago I had a 220swift rebarreled to 6mm Remington. I chose a 1in 9 twist. When I tried to shoot some 70gr hollowpoints things went sideways. Really at close range bullets keyhole IF they hit the target. I am pretty sure some of them flew apart. Now with 100, and 105 gr bullets it is a tack driver
That is not uncommon w/ some varmint bullets in .223/5.56 1/7 twist, especially new barrels. It's not the weight or the length of the bullet at issue viz the twist, it's the construction of the bullet. I've had that happen w/ a 1/7 AR and 55 grain Hornady SX bullets. I shoot a lot of varmints and after that experience switched to the 55 SP and thousands later, no issues at all. In your rifle a stronger 70 may have shot very well.
Our government hunter here uses a 6mm wildcat for coyotes and lion and uses a 55 grain bullet run FAST. He has no issues and it kills yotes very well indeed. I've witnessed his work on my ranch here and while I never thought about a 55 grain in 6mm I can attest to its effectiveness.
That may be true, but I have to wonder what twist he is running with 6mm 55 gr bullets. 55gr 6mms would be considerably shorter than .224 55 gr bullets. Less surface engagement on the rifling
Another thing I should mention is I was getting LOT of copper fouling. Plus, coyotes are easy to kill. I have used 55gr FMJ from a 223 and it does the job. I also have think that a heavier constructed 75 gr bullet might work in my 6mm with the 1in 9.
The rpms a bullet spins if twist rate times fps. A 3000fps bullet fired in a 1-12 barrel spins one turn every ft and at 3000x60second it is spinning 180,000rpm. A hot 6mm with a 1-9 firing a 3500fps 75 gr bullet is spinning 1.33 turns per foot and at 3500fps thats 4,667 turns per second now take it by 60seconds and you got 280,000rpm and it takes a tough bullet to both stand up to rifling engagement and keep it integrity once it leaves the barrel.
Another thing I should mention is I was getting LOT of copper fouling. Plus, coyotes are easy to kill. I have used 55gr FMJ from a 223 and it does the job. I also have think that a heavier constructed 75 gr bullet might work in my 6mm with the 1in 9.
The rpms a bullet spins if twist rate times fps. A 3000fps bullet fired in a 1-12 barrel spins one turn every ft and at 3000x60second it is spinning 180,000rpm. A hot 6mm with a 1-9 firing a 3500fps 75 gr bullet is spinning 1.33 turns per foot and at 3500fps thats 4,667 turns per second now take it by 60seconds and you got 280,000rpm and it takes a tough bullet to both stand up to rifling engagement and keep it integrity once it leaves the barrel.
AJ
US Veteran
As long as an AR shoots "Minute of Chest" at 100 yards or so is all that matters.
Yes, that's the point.
Bullet construction matters.
If you are having bullets disintegrate in flight, that's a matter of bullet construction, velocity and twist rate, not just twist rate. Use a tougher bullet and the disintegration stops.
I just emailed him. 1/10. And there you go. He's contemplating a 1/8 and will see how those bullets perform.
55gr Nosler Ballistic Tip.
We'll see how it works with a tighter twist I suppose.
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Read "Blackhawk Down" and the 62gr green tip standard A2 round would hit a "Skinny" once twice three times and he'd just keep on going. The soldiers wanted the M14 being used by the D Boys which put them down with one hit every time. The 55 gr was very deadly at 1/12 in VN and with excellent accuracy.
That's a pretty broad statement. We used WCC M193 ball through the '90s in a few hundred Colt HBARs (20" bbl), 1:7 twist, with excellent training and qualification results. The times the ammo was used in shootings it was always decisive.
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You mean shooting a man in the hand with a .45 Automatic won’t knock him down?
One can kill a man with a 9mm, but with a 45 ACP, you also kill his soul.
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I dont really care to know. i know i have a whole lot of fun on the range. Twist rates or ballistics weren't mentioned at the carbine course i attended. You make do with what you have is what the line did.
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