AR guys: 1:7" and 1:9"

[benzy2]

I've tried 75gr Amax bullets in a 1:9" twist and they didn't stabilize. Out of that same barrel both 75gr Hornady HPBT Match and 77gr Sierra Match King both shot great. The 75gr Amax also was too long of a bullet to load in a magazine and was strictly used for slow fire portions of Service Rifle matches. As you said, I wouldn't hold out much hope for the Amax in a 1:9" twist, but that is an ultra long for weight bullet, more so than many of the other common mid 70gr bullets out there.

 

[Bushpilot]

Barrels don't wear out form the projectiles going down them. Imparting spin on a bullet does nothing for wear.

Not exactly true. While rate of twist may not significantly contribute to throat erosion, where most barrel wear occurs, (although, a faster twist does raise pressures slightly), bores and rifling do wear, it just isn't where the majority of the barrel wear will occur first. The faster the twist, the more wear.

It is also interesting to note that no one has bothered to mention velocity in this discussion. I suppose that maximum velocity within pressure limits is assumed. However, remember that velocity is just as important as twist or bullet length (weight). Although twist is described as the distance, expressed in inches, required for the bullet to make one rotation, the important factor really is the number rotations over a period of time for a given bullet length. So, if you speed the bullet up, less rate of twist is required. Slow the bullet down, more twist is needed to stabilize it. The guys who shoot muzzleloaders (at least the ones that shoot them well) deal with this stuff more directly in projectile and load selection than most other shooters do.

There is also such a thing a too much rotation or "over stabilized" and this diminishes accuracy." A 65 grain bullet fired in a 1:9 AR at 3000 fps is rotating at 240,000 RPM or 4000 rotations per SECOND. At these kinds of rotational speeds it is easy to see how any minor imperfection in the construction of the bullet will have negative effects on accuracy. This is one of the reasons why you want just enough rate of twist to stabilize the bullet but no more than you need because it magnifies bullet imperfections (among other things). MV x (12/twist rate in inches) x 60 = Bullet RPM

In conventional copper jacketed, lead core construction bullets right at 70 grains (give or take one or two grains) is the maximum most 1:9 twist barrels will stabilize.

1:9 is actually a pretty fast rate of twist and probably the most versatile for an AR. Remember that it's not the weight of the bullet that matters; it’s the length and shape. 1:9 will actually stabilize most heavy (or better stated, longer) bullets pretty well, under most conditions, and is much better with the lighter (shorter) stuff.

Bottom line: 1:9 is probably the best bet for the vast majority of AR shooters. There is such a thing as too much rate of twist. The only reason to opt for a 1:7 or 1:8 is if you are exclusively shooting the longest (and slowest) bullets possible.

There is some pretty good information on the subject here:

http://www.armalite.com/images/Tech...OJECTILE WT vs TWIST 120725 Rev 0 (Final).pdf

Just remember, that Greenhill’s equation actually is a little conservative, leaning a little bit on the "faster" rate of twist side to compensate for variable atmospheric conditions. You can usually get by with a little less rate of twist than his equation states.

Another interesting article is:
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

 

[ugaarguy]

Remember that it's not the weight of the bullet that matters; it’s the length and shape.

No kidding. If I hadn't known that I wouldn't have written:

Many folks also fail to understand that alternate construction bullets may be shorter for their weight (PRL / DRT powdered tungsten-tin core, uncommon), or longer for their weight (Barnes, Hornady, etc, copper solids, very common).

1:9 will actually stabilize most heavy (or better stated, longer) bullets pretty well, under most conditions, and is much better with the lighter (shorter) stuff.

Extensive testing documented in US Army FM 23-9 contradicts your assertion. M193 Ball is well made ammo, but it's far from being match grade. If a 1:7 twist will shoot M193 just as accurately as a 1:12 twist, what makes you think 1:9 is so much better than 1:7 for short projectiles?

Bottom line: 1:9 is probably the best bet for the vast majority of AR shooters. There is such a thing as too much rate of twist. The only reason to opt for a 1:7 or 1:8 is if you are exclusively shooting the longest (and slowest) bullets possible.

Unless you're shooting at extremely long ranges (like over 1k yards), or shooting really poorly made bullets, excess twist is not even a factor. We're talking about general purpose rifles here, not artillery pieces (where you actually do have to factor in twist rate to the aiming equation).

1:7 twist will stabilize a far wider range of projectiles across far broader conditions than 1:9. If you want to shoot large amounts of extremely short, thinly jacketed bullets then you're better off with a 1:12 or even 1:14 twist.

The real bottom line is that 1:7 and 1:8 are more versatile than 1:9.

As far as what rate of twist is needed for your application, a quality barrel that is indeed a 1:9 will shoot pretty much anything that will fit in the magazine. That includes the 62gn TTSX Barnes bullet. Barnes recommends a 1:8 but the Miller index for that bullet at 2700fps on a cool day at sea level is a 1.12 which is stable.

The Miller Index - a theory. Barnes Bullets minimum twist recommendations - the result of putting that theory into practice, and finding, in the words of Jan L. A. van de Snepscheut, "In theory, there is no difference between theory and practice. But, in practice, there is."

 

[CountryUgly]

My Colt has a 1:7 twist. All I shoot is 55gr federal bulk and Hornady steel match 75gr. Point of impact is a little different but groups are both MOA or better. Just MOE so take it for what it cost ya

 

[Bushpilot]

No kidding. If I hadn't known that I wouldn't have written:

Wow ugaarguy, that was a bit of an unwarranted rant, especially since I wasn't even responding to anything you posted. I was responding to the OP, but since you brought it up:

Extensive testing documented in US Army FM 23-9 contradicts your assertion. M193 Ball is well made ammo, but it's far from being match grade. If a 1:7 twist will shoot M193 just as accurately as a 1:12 twist, what makes you think 1:9 is so much better than 1:7 for short projectiles?

Ugaarguy, as far as your US Army FM 23-9 "evidence" is concerned, what it actually says is "The M16A2 rifle with its 1:7 twist fires both types[sic] ammunition [M855 and M193] with little difference in accuracy to a range of 500 meters". It doesn’t say a 1:7 twist is “just” as accurate as a 1:12 with M193. So, if you’re going to go on a rant and cite sources, at least quote them accurately. But, if you don’t believe that too much rate of twist is detrimental to accuracy then I suggest you go chat with some unlimited benchrest shooters on the topic of “rate of twist” and try telling them that

excess twist is not even a factor

and how great 1:7 is with short, bullets.

To answer your question of "why I think the slower rate of 1:9 is better: 1. The difference between the 1:7 twist that you apparently favor and a 1:12 for example costs about 100fps, 20 fps per inch roughly. 2. The faster rate increases pressures and wear (although only slightly). 3. Excess rate of twist IS detrimental to top accuracy, all else being equal. If you think excess rate of twist is not a factor than why don't barrel/rifle makers just rifle EVERYTHING at 1:7 or why not 1:5 even? 4. The main reason 1:9 is preferable in my opinion for the average AR owner is that he is going to be shooting a mixture of milsurp and sporting ammo of various weights, not just the longer heavier bullets. There are plenty of factory sporting loads out there with bullets even shorter (lighter) than M193's 55gr. ball, and a slower 1:9 twist is superior with shorter (lighter) bullets if the best possible accuracy is the goal, while still enough to stabilize all but the longest bullets.

 

[helotaxi]

Ugaarguy, as far as your US Army FM 23-9 "evidence" is concerned, what it actually says is "The M16A2 rifle with its 1:7 twist fires both types[sic] ammunition [M855 and M193] with little difference in accuracy to a range of 500 meters". It doesn’t say a 1:7 twist is “just” as accurate as a 1:12 with M193. So, if you’re going to go on a rant and cite sources, at least quote them accurately. But, if you don’t believe that too much rate of twist is detrimental to accuracy then I suggest you go chat with some unlimited benchrest shooters on the topic of “rate of twist” and try telling them that

DO you understand the game tha tbenchrest shooters are playing with regard to rate of twist? Based on your argument I would say that you do not.

There are plenty of factory sporting loads out there with bullets even shorter (lighter) than M193's 55gr. ball, and a slower 1:9 twist is superior with shorter (lighter) bullets if the best possible accuracy is the goal, while still enough to stabilize all but the longest bullets.

The difference in potential accuracy between a 1:9 and 1:7 barrel assuming that both are select match grade barrels with perfectly cut chambers and perfect crowns is so miniscule that ONLY a top level benchrest shooter can appreciate it. The reason that benchrest shooters shoot the slowest twist barrel that they can get away with is to minimize torque on the rifle from imparting spin on the bullet. The less the torquing force, the less the rifle is upset in the bags, the more consistent the shot-to-shot precision is. With good bullets the actual difference in precision between the two twists in question is measurable in hundredths of an inch. Arguing that anyone other than a benchrest shooter can appreciate that difference is ridiculous.

ugaarguy- The Miller index is theory, but so is gravity. Gravity is only slightly more backed by empirical evidence. Barnes recommends a 1:8 for the same reason that the US Army went with a 1:7: margin of error. The 1:7 was chosen by the DoD based on the Miller index, BTW. They insisted on a significant buffer (don't remember the number specified) under the most adverse conditions (sub-zero at sea level) for the 64gn tracer.

 

[NeuseRvrRat]

so what twist do the benchrest shooters shooting ARs use?

 

[baylorattorney]

1.7 causes more damage as it has a better chance of tumbling where as 1.9 is a zinger, hole puncher, more accurate tho less lethal.

 

[NeuseRvrRat]

 

[helotaxi]

so what twist do the benchrest shooters shooting ARs use?

Exactly!

 

[helotaxi]

1.7 causes more damage as it has a better chance of tumbling where as 1.9 is a zinger, hole puncher, more accurate tho less lethal.

The opposite is shown to be true. The higher rate of twist usually allows for slightly better penetration. Regardless, the bullet plays a much bigger role than spin with regard to the likelihood of a bullet tumbling on impact with soft tissue and the subsequent damage as well.

 

[Bushpilot]

DO you understand the game tha tbenchrest shooters are playing with regard to rate of twist? Based on your argument I would say that you do not.

The less the torquing force, the less the rifle is upset in the bags, the more consistent the shot-to-shot precision is. With good bullets the actual difference in precision between the two twists in question is measurable in hundredths of an inch. Arguing that anyone other than a benchrest shooter can appreciate that difference is ridiculous.

The difference in potential accuracy between a 1:9 and 1:7 barrel assuming that both are select match grade barrels with perfectly cut chambers and perfect crowns is so miniscule that ONLY a top level benchrest shooter can appreciate it.

First off, torque is only one reason why the least or slowest rate of twist used to adequately to stabilize a bullet the better. Torque however, was NOT one of the reasons that I was referring to.

Second are you saying that I am somehow wrong because I am only right by a small amount, "hundredths of an inch?" Now THAT is ridiculous.

And lastly I totally disagree that the differences between 1:7 and 1:9 (over 20%) are so small and insignificant that they can ONLY be appreciated by benchrest shooters, especially if I can vary the velocity or bullet length. I have loaded for numerous cartridges where by varying the velocity and bullet length (weight), I have gone up to and beyond or fell below a barrels "sweet spot" or most effective "zone" that results from rate of twist limitations. I have done this with short, light bullets and high velocities as well as with long, heavy bullets and low velocities, in sporter weight rifles, where the difference was obvious, sometimes dramatic and was more than theoretical. Any handloader with any experience has had similar results. The 30/06 is a good example. The 06 typically has 1:10 to stabilize the long 200 and 220 grain bullets. The 308 by the way typically has 1:12 because you can’t load the heavier bullets without seating the bullet too deep. The 06 will shoot 110 grain bullets but the accuracy is almost never equal to what the 150, 165 or 180gr bullets deliver or the 110 and 120gr bullets in the 308. The short bullet is over stabilized (which is better than under stabilized) but I certainly don’t need a benchrest rifle to see the effects of too fast a rate twist.

 

[helotaxi]

You're attributing it to twist rate but have no way of knowing that is the actual difference. It's actually pretty well known that large cases and light bullets are not the best recipe for accuracy because of pressure and burn inconsistencies. There are plenty of shooters out there with 1:10 .308s as well, myself included. I haven't bothered with the heavier bullets because I haven't needed them, but it's lights out with 155s and 168s.

Despite all the arguing back and forth on the internet, I've yet to see any evidence showing that there is a PRACTICAL difference in accuracy between a fast twist and slow twist barrel and if there was, it would manifest itself with the heavy bullets just as much as with the light bullets. I've also seen way too many 1:7 ARs that shoot bugholes with 50-55gn bullets to accept that it makes a difference that you could possibly realize in that platform. I know plenty of match shooters that use 52gn MatchKings for short range practice with their 1:8 competition rifles. If the difference in accuracy was that great, they would make lousy practice rounds since the shooter wouldn't be able to trust the feedback they were getting from the rifle. Sorry, practical difference in accuracy from a "too fast" barrel twist is a myth.

 

[ugaarguy]

Bushpilot, I was just responding to where you quoted me. I'm sorry you perceived it as a rant. That was truly not my intent.

Helotaxi, I hope nothing I posted offended you.

To both of you, thank you for debating the issue with me, and understanding that while our discussions here on THR often get intense, I'm not taking anything personally. I think almost everyone who's posted thus far has contributed to a wealth of information in this thread. I think we've given the OP quite a bit of information on the question asked. I'm now going to bow out of this thread, because I don't know how much more I can add. I am going to read more about the points both of you have touched on. I'm sure quite a bit of it will go into my saved information on the topic. I'm not a benchrest shooter, but even so I'm confident that much of what's been discussed here will help me to further improve my handloading. Thank you for increasing my knowledge, and challenging my thinking in such a constructive manner.

 

[helotaxi]

If you haven't already read it, let me recommend Applied Ballistics for Long Range Shooting by Brian Litz. He's and aero engineer type, national champion long range shooter and currently the Chief Ballistician for Berger Bullets. It is a wealth of knowledge distilled into layman's terms as opposed to being full of formulas and proofs and otherwise generally written for engineers. He dedicated an entire chapter to the topic of spin and rifling rates.

And I don't get offended. I might think someone else is and idiot, but it doesn't offend me. And I don't mean anyone in this thread by that.

 

[Bushpilot]

ugaarguy, I really wasn't intending to dispute what you wrote in that first quote, although it may sound that way when I go back and read it. I actually agree that a little over 70 is about the limit for 1:9. My comments really were directed at the OP, not in trying to "inform" or "correct" you. Actually, I almost took it out when I first wrote it but was just trying to expand on it in my comments to the OP. I honestly didn't even realize it was you that I quoted until just now when I went back to look. Thank you for your comments.

HELOTAXI, your right in as far as I can't PROVE that the rate of twist is the reason 110 grain bullets won't shoot as well in my 1:10 30/06 as they do in my 1:12 308. I haven't conducted a scientifically valid experiment as there are far too many variables. However, the fact that accuracy improves in the 06 when I drop the velocity down is a good indicator. I also doubt that many of the opinions expressed on this board are backed up by scientifically valid experiments conducted by the posters. I nevertheless believe that I have personally witnessed the effects of too high a rate of twist for top accuracy numerous times in various rifles. I also think the main reason benchrest shooters don't use super fast 1:7 rates of twist is that it isn't optimal for reasons other than "torque". If there are guys shooting good groups with 1:7/1:8 and 50-55 gr bullets in 5.56/223 maybe they are dropping the velocity down or maybe they would be shooting even better groups with a slower twist. Either way, at top velocities, that rate of twist is not optimal for that length (weight) bullet as any custom barrel makers recommendation chart will show.

I've yet to see any evidence showing that there is a PRACTICAL difference in accuracy between a fast twist and slow twist barrel and if there was, it would manifest itself with the heavy bullets just as much as with the light bullets.

Please don’t take this the wrong way and I may be completely off base here but the last part of this statement makes me question whether you really understand Greenhill’s formula. If this is the case I might have a few links or pdfs...

 

[Bushpilot]

Ha Helo.. we both had the same idea.... at the same time....lol Maybe we could both step back and read a little more...

 

[NWcityguy2]

Thanks guys it has been very informational. I'm a gun guy and avid reloader so most of the book knowledge I am aware of. The most interesting thing I've learned though it that most of the longer bullets that benefit from 1:7 aren't designed to be loaded to magazine length anyway.

 

[baylorattorney]

The opposite is shown to be true. The higher rate of twist usually allows for slightly better penetration. Regardless, the bullet plays a much bigger role than spin with regard to the likelihood of a bullet tumbling on impact with soft tissue and the subsequent damage as well.

I agree hollow points make a big difference and the higher the twist the deeper the penetration.

 

[Caliper_Mi]

Well, just to add some numbers here, I went downstairs and measured two Hornady bullets.

The 75gr BTHP match (not A-max) is 0.99" long
The 68gr BTHP match is 0.979" long.

When I plug those lengths into JBM's stability calculator with realistic velocities (esp for a 16" barrel) the 1:9 looks iffy as the temperature falls or as distances get longer and velocity falls.

 

[helotaxi]

as distances get longer and velocity falls.

It doesn't work that way. Velocity decays, true, but the velocity used the calculations is used to determine the RPM imparted on the bullet by the rifling. As the velocity decays because of range, the RPM decays at a slower rate and the effect is the same as if the barrel had a faster twist. The Miller number actually increases with range.

Velocity, in and of itself, is actually the enemy of bullet stability because the center of drag is forward of the center of gravity so the higher the velocity, the more force is acting on the CoD. RPM is needed to overcome that force. The good news is that for a given barrel twist, the increase in upset force is more than offset by the increase in stability from the increase in RPM. As velocity decays because of drag, the upset force is reduced, but the RPM is relatively constant by comparison. The result is that the bullet becomes increasingly stable as it travels downrange.

 

[helotaxi]

Please don’t take this the wrong way and I may be completely off base here but the last part of this statement makes me question whether you really understand Greenhill’s formula. If this is the case I might have a few links or pdfs...

The Greenhill formula is best used for what it was designed to model: short for caliber, blunt ogive, flat based artillery shells. It has proven marginal or worse for describing the stability of a long for caliber, boat-tailed high velocity rifle bullet. Very similar in that regard to the G1 vs. G7 ballistic model.

And the guys getting good groups with fast twist barrels and light bullets aren't downloading at all. They are simply matching a good barrel with a good bullet. The only detrimental effect that a faster than required (the idea of "optimal" is misguided and barrel makers that I've seen list their rates of twists as "up to XX bullet weight" rather than bracket it) barrel twist has is to exaggerate any imperfections in the bullet's weight distribution and amplifying the wobble that results from the center of gravity of the bullet being somewhere other than along the rotational axis of the bullet. The very same problem will occur if the bullet leaves the case and enters the rifling with some amount of yaw. This can result in a divergent oscillation and the bullet tumbling at longer range (independent of the transonic transition). Such bullets shoot poorly out of barrels of any twist. Quality bullets of modern manufacture are nearly perfectly balanced and the result is that they shoot extremely well regardless of rate of twist. Benchrest shooters shoot expensive, short for caliber, flat based bullets. Short for caliber and the flat base reduce the chances of having a manufacturing flaw in the first place and the bullets are expensive because of the level of precision in the machines used to make them and the level of quality control to ensure bullet consistency. Benchrest guys take this a step further and spin them as little as possible to reduce any effects from the otherwise negligible imperfections in these bullets and also to reduce reactionary torque to the lowest amount possible.