Hey all-

I know 1 in 7" and 1 in 8" twist barrels have become somewhat standard in the AR world.

My question is: Is there anything wrong with a 1 in 9" twist for general range use and HD duty? I don't think there is but I find myself hesitant to jump at a good deal with a 1 in 9" barrel.

What say you? Go for it or spend more to "get with the century"

Bruce

I thought 1:9 was never a standard anything. In fact, it's the minority.

1:7 is what's in the military barrel, isn't it? I'm no expert or anything even close, but what I read is that the 1:7 barrel is better for heavier bullets.

The faster the twist the more stable the bullet as the yaw is kept tight during flight. The 1/7 twist of the M-16A2 and everything since didn't need the fast twist to stabilize the 55 grain bullet that was common until the M855/SS109 which is 62 gn. We were issued this in the mid 80's. Tt was fast to stabilize the tracer rounds which I believe were about 69 gn if I remember correctly. The A1 had a much slower twist at 1/14. This is why the A1 was considered so deadly if you were hit with the 55 gn round fired from this rifle. The yaw was much more loose, and went unstable when it hit anything, including the human body causing significantly more trauma than say a 55 gn FMJ fired from a 1/7 twist. As heavier bullets become the norm, the fast rate is needed to keep the yaw tight which allows for greater accuracy.

I have a couple older AR that are 1 in 9. If you are mostly shooting 55 Gr and 62 Gr bullets they seem to shoot fine. I also shoot some varmint loads out of them every year such as 45 Gr and 50 Gr and they shoot real well.

Personally, I think we sometimes make too big of a deal out of barrel twist for general purpose rifles. If you read the mellonics report "Analyisis of M16A2 Rifle Characterisitcs and Recommended Improvements"(FEB86), the Army actually found the 1/9 twist to be more accurate than the 1/7 with the service rounds and made the case that 1/9 was better overall. However, this was before the MK262 77gr OTM, so this data is dated and possibly OBE.

If all you are going to do is feed your rifle Walmart special 55-62 FMJs or JHPs out to 100 yards you probably aren't going to see much impact from the barrel twist. Remember you are stacking tolerances. Your ability as a shooter will probably affect the tightness of your groups more than the twist will.

If you are shooting precision rifles, that's a different story. You should do some research and pick the best twist and round combination for you desired purpose.

Both my 5.56s are 1-9.
When I finally shoot out a barrel, I'll buy a faster twist maybe but until then it works just fine for me.
Anything from 40-69grs works in my 1/9ers. YMMV.

Rifling twist rate affects shooting precision (small groups) two ways.
As already mentioned, if the twist rate is too slow, the bullet will not be stabilized sufficiently and at some point in its flight will tumble, lose velocity quickly, and fly off course.
What is not generally recognized, however, is what Bryan Litz discusses in his series of books, including Applied Ballistics for Long-Range Shooting.

No matter how carefully manufactured, all bullets are imperfect. The variations are usually very tiny, but they nevertheless exist, and poorer quality bullets are more imperfect. The relationship between bullet imperfection and rifling twist rate is that the faster the bullet spins, the greater effect the imperfections have on precision. That’s why precision benchrest shooters at close ranges (e.g., 100 yards) prefer slower rifling twists. Long range shooters on the other hand use barrels with faster twists to ensure their bullets remain stable at extended distances; if a bullet tumbles in flight before reaching its target, it doesn’t matter how precise it may be at shorter distances.

But what does all this have to do with an average AR-15 used for shooting at close ranges (within a few hundred yards) for recreation (fun) or defensive purposes? According to Litz’s data, a 1/9" rifling twist rate will provide marginal or less stability to most 69 grain bullets under some conditions. And the heavier (longer, actually) the bullet is beyond that, the less suitable 1/9" will be. A 1/9" twist rate isn’t ideal even for the Lake City 62 grain M855 bullet; according to Litz, the twist rate should be 1/8.7" or faster for optimum performance. In practice, though, I’ve never seen any problems using M855 in a 1/9 barrel.

If we’re not concerned about long range stability with extra heavy bullets or a high level of precision, however, I cannot imagine any practical difference between 1/9 and the more common 1/7". Is there a theoretical difference in lethality if bullets are fired from one or the other? Perhaps, but it must surely be so small as to not be a matter of concern either.

quote:

Originally posted by RNshooter:
My question is: Is there anything wrong with a 1 in 9" twist for general range use and HD duty?

It all depends upon the bullets you wish to shoot from the rifle. My first AR had a 1/9 twist 16" barrel. It was a quality match-grade barrel and it shot match-grade bullets up to 69 grains with great accuracy. On hot days at high altitude it shot some heavier bullets accurately, but the results weren't consistent. When that barrel was shot out, I replaced it with a similar 1/8 barrel and I am pleased with my decision.

My 223 barrels now have twist rates of 1/8, 1/7.7, or 1/7.

IMO the best 223 bullets for any purpose are the heavier ones. Sure, there's no beating the cheap cost of 55 FMJ bullets, but other bullets perform better. I see no reason to have a barrel that is optimized for lighter and less functional bullets.

Most barrels are relatively inexpensive, especially considering the amortized cost per round over the barrel's useful life. 1/9 barrels tend to be on sale for a reason. Get a barrel with a twist between 1/8 and 1/7.

quote:

Originally posted by sigfreund:
No matter how carefully manufactured, all bullets are imperfect. The variations are usually very tiny, but they nevertheless exist, and poorer quality bullets are more imperfect. The relationship between bullet imperfection and rifling twist rate is that the faster the bullet spins, the greater effect the imperfections have on precision. That’s why precision benchrest shooters at close ranges (e.g., 100 yards) prefer slower rifling twists.

In addition, some lightweight, especially thin-walled varmint bullets can tear themselves apart if fired through a barrel with too tight of a twist.

So there are a few situations in which a slower twist rates would be preferable.

quote:

Originally posted by RogueJSK:

quote:

Originally posted by sigfreund:
No matter how carefully manufactured, all bullets are imperfect. The variations are usually very tiny, but they nevertheless exist, and poorer quality bullets are more imperfect. The relationship between bullet imperfection and rifling twist rate is that the faster the bullet spins, the greater effect the imperfections have on precision. That’s why precision benchrest shooters at close ranges (e.g., 100 yards) prefer slower rifling twists.

In addition, some lightweight, especially thin-walled varmint bullets can tear themselves apart if fired through a barrel with too tight of a twist.

So there are a few situations in which a slower twist rates would be preferable.

Have you actually observed a bullet tear itself apart? I've read the same anecdotes on line, but I have never seen it happen in real life.

quote:

Originally posted by CD228:

quote:

Originally posted by RogueJSK:
In addition, some lightweight, especially thin-walled varmint bullets can tear themselves apart if fired through a barrel with too tight of a twist.

So there are a few situations in which a slower twist rates would be preferable.

Have you actually observed a bullet tear itself apart? I've read the same anecdotes on line, but I have never seen it happen in real life.

I have not seen it myself, but have buddies who claim it has occurred.
The first was cheap, very light weight 223 bullets (40 grains or less) in a long barrel 223 bolt action rifle, with a 1/7 twist. Muzzle velocity was likely in the 3300 to 3400 fps ballpark, maybe more.

The second was a thin-skinned 7mm heavy match bullet pushed to 3200+ fps in a long barreled magnum chambering.

Neither of these situations apply to the OP.

I've shot Hornady's 40 grain Vmax loads in a 24" 1/8 barrel and in a 18" 1/7 barrel. Velocities were up there, which means bullet spin rates were high. The bullets flew just fine.

The OP can get by with a 1/9 barrel if lighter bullets are used. But there's no reason for it. Get a 1/8 or faster twist barrel.

quote:

Originally posted by CD228:

quote:

Originally posted by RogueJSK:
In addition, some lightweight, especially thin-walled varmint bullets can tear themselves apart if fired through a barrel with too tight of a twist.

So there are a few situations in which a slower twist rates would be preferable.

Have you actually observed a bullet tear itself apart? I've read the same anecdotes on line, but I have never seen it happen in real life.

I have never seen or heard of any .224 caliber bullet self-immolating out of a rifle. I have seen several instances of 6mm bullet spontaneously disintegrating in flight. It's been several years now, but I had a friend who was competing in F-class at a very short 300 yard match with his beloved 6mm bolt action match rifle. He was trying out some new bullets; pretty short light ones. He started shooting his first string and after he fired the target would not go down because there was not bullet impact on the berm. He took another shot and that one registered. Then he took yet another and nothing. After a few more rounds, I noticed that a little white puff of smoke in the form of a cylinder was appearing about 75 yards down range. A few more rounds and it was confirmed. His bullets were disintegrating in flight. He had a very fast twist in the barrel and the bullets were quite short and light because we were shooting at just 300 yards; no need for the long heavies.

Needless to say, he quit shooting for the day.

The 6mm seems to be the only one I ever heard/seen do that.

Also, Bryan Litz is a big proponent of very fast twists for long range purposes. His findings which he published a few years back were that the higher than normal spin rate actually caused the bullet to retain its BC, or put another way; not shed its BC as quickly, during flight.

So a lot of long range F-Class shooters increased their twist rates some.

Disintegrating bullets used to be discussed in hunting magazines long, long ago, but the only cartridge I ever saw the phenomenon associated with was the 220 Swift: light varmint hunting bullets driven at screaming velocities. In those days, though, the rifling twist rates for the round would not have been particularly fast; probably 1/12 or 1/14". The comments even then seemed to be much more than vague anecdotes.

quote:

Originally posted by CD228:
Have you actually observed a bullet tear itself apart? I've read the same anecdotes on line, but I have never seen it happen in real life.

Not personally. Like you, I've only read about it online, including (IIRC) reports from an avid varmint shooter here on the forum (3/4 Flap, perhaps?).

As noted, it would seem to be an issue primarily with the combination of varmint bullets with tight twist rates and high velocities, so probably not as likely with .223.

Way back before plastic tip varmint bullets came out. I was testing some handloads for varmint hunting with a Hornady 50 gr SX out of my 22-250 with a 1 in 14 twist. To my surprise only 2 of 5 or 3 out of 5 bullets hit the paper with some strange looking cuts in the paper. During further testing I noticed a gray mist forming slightly smaller than a foot on some shots, around 70 - 85 yards out. I was a little slow in making the connection before it hit me. Some of the bullets were disaggregating in the air. These days plastic tip varmint bullets are the way to go for varmint hunting.

The only other time I ran into this was shooting was with a Hornady 162 gr plastic tip match bullets out of a 7mm STW in a 26.5" barrel with a 1 in 9 twist.

As someone wrote above, it depends on the weight bullets you plan on shooting. My AR's are varmint guns so I like the slower twists for better accuracy with lighter bullets. I have a couple couple 1:7 guns, but prefer 1:8 or even 1:9. My 223 bolt rifle is 1:12 and shoots 50 to 55 grain bullets extremely well.

I tend to shoot 45-69 gr bullets. If I do want to shoot long and heavy specialty bullets I will shoot them in one of my 1 in 7 barrels.

Bruce

my short range AR, (service Rifle) will put 69gr SMK's inside an inch at 100,

I shot with some folks that did good with 75gr, but the 2 barrels I shot would not,

all are 1:9

quote:

Originally posted by lyman:
my short range AR, (service Rifle) will put 69gr SMK's inside an inch at 100,

Every barrel I have owned shoots quality 69 SMK loads well. Twists of 1/9 through 1/7, barrel lengths of 14.5" through 24". Chambers of 5.56, 2.23, and Wylde. I think it's just a great bullet design, and I suspect it's pretty tolerant of jump/jam to the lands. For some of my barrels, 69 SMK loads produced the tightest groups of all loads at 100 yards -- bugholes which appear to have been shot by bolt actions. It's unfortunate that the 69's G1 BC is so low at .301.

The match bullets in the 73-77 grain class offer noticeably superior GI BCs of around .400, but they can be finicky in some barrels. Especially those with 1/9 or slower twists. Hornady's 75 BTHP and Sierra's 77 MK are widely used with good results. Hornady's 73 ELD-M seems to like tight chambers and/or limited jump.

For lighter bullets, Hornady's 55 Vmax tends to be accurate in barrels without regard to twist rates. Its G1 BC of .255 is unspectacular.