View Full Version : Twist rate and barrel wear
The following is extracted from "Black Rifle II", chapter 1:
"Many experts claimed the 1 turn in 9" would have been much more desirable for many reasons, primarily longevity and durability. The much faster 1-in-7" twist caused premature wear, and was found to be significantly more likely to burn out barrels on automatic/burst fire. Additionally, the 1-in-7" twist definitely overstabilizes the 55-grain M193 ball projectile, degrading it's terminal performance."
So, almost 20 years after this book was published, is this still seen as holding true?
I'll add another one to your original question.......Why are there an increasing number of 1:8 on the market today, when they never used to be offered?
I shoot primarily 77gr reloads so I opt for the faster twists.....as per what appears to still be consensus. Will be interesting to see what others have to say on this topic.
This will likely end up in a farting match about M855 and bearing surface of different bullets.
None of that is true.
There is no such thing as “over stabilization”, inconsistent FMJ terminal performance is caused by AOA and fleet yaw.
Twist has a minor impact on barrel life, it’s mainly a concern for artillery tubes. I’m aware of some theories being talked about, but I’m not aware of any actual testing being done.
Heavier bullets have proven to be better for longer range use, as well as terminal performance. Arguments based on M193 performance are seriously dated, in addition to being factual incorrect.
Ya, I’m in no way convinced that it was true when the book was written.
I think that the quality of the manufacturing in the barrel process matters a whole lot more on longevity and durability then whatever twist rate you have.
Sounds like some good ole fashioned Fudd nonsense that seems to permeate gun culture.
The author of the book may have referenced the Army's 1986 study on the M16A2, in which the Army's dislike of the A2 was clearly evident. One excerpt:
"While any alternative must be tested, previous firing tests have confirmed that a 1:9 twist will provide for stability of a bullet similar to the new ball round. Reducing barrel twist to 1:9 will result in less stress on the bullet, barrel life will be improved, and barrel fouling will be reduced. While this twist may not fully stabilize the tracer round to maximum range, the rifle tracer is normally used as a marking round at extended ranges and precise accuracy is not required. The M16A1 has one twist (rifling) for each 12 inches of barrel length and it has very effective terminal ballistics against personnel targets. It is generally accepted that less bullet stability will enhance terminal ballistics. Therefore, the increased twist of the M16A2, one twist in seven inches, should be tested against a one in nine twist barrel, which would probably produce better terminal ballistics against personnel targets.
A very important consideration is that reducing twist to 1:9 will probably improve accuracy at all ranges, particularly at 25 meters and in the primary range band out to 300 meters. A general rule is that minimum twist should be used to stablilize the round. Any additional twist will increase variability, causing the bullet to move in a corkscrew-type pattern at closer ranges."
Of course, this was before the advent of rounds such as the Mk 262.
https://apps.dtic.mil/sti/pdfs/ADA168577.pdf
Lysander probably has some information relevant to this conversation…..and I’d be quite surprised if it matches what’s been stated here.
I'll add another one to your original question.......Why are there an increasing number of 1:8 on the market today, when they never used to be offered?
Manufacturers are getting smart??? Seems unlikely, but it's possible. 9 twist in a 20 inch barrels works fine for us all the way to 80 gr Noslers.
I'm fine with 7 twist, but 8 really is the right answer since we don't shoot M856 tracers.
fedupflyer
10-24-23, 22:54
None of that is true.
There is no such thing as “over stabilization”, inconsistent FMJ terminal performance is caused by AOA and fleet yaw.
Maybe if you cast your own bullets.
If they a copper jacketed, then yeah nothing worry about.
Maybe if you cast your own bullets.
If they a copper jacketed, then yeah nothing worry about.
I’ve exploded super light varmint bullets in longer, extra fast twist barrels. Fun fact, the bullets that didn’t come apart ended up in a very nice tight group.
Over stabilization in respect to terminal performance is a myth. Tissue is so much denser than air, that no amount of stability factor anywhere near the realm of what we are talking about here, could keep a bullet stabilized.
Lysander probably has some information relevant to this conversation…..and I’d be quite surprised if it matches what’s been stated here.
I haven't chimed in as Todd had covered it pretty well.
As to "Analysis of M16A2 Rifle Characteristics and Recommended Improvements" . . . . (the above quoted report)
In the mid-1980s there two factions in the Army, one felt that the M16A1 had overcome all of its teething troubles and was now a mature and reliable system, and in a few years, the poor reputation of the early 60s would be replaced as more soldiers used and saw that it was a reliable system. And to replace it with something different at this point would give the implication that the M16A1 was still deficient in reliability or durability.
The other faction believed that the M16A1 had overcome all of its teething troubles and was now a mature and reliable system, BUT, still had a "perception" problem with the troops, as they perceived it to be fragile and unreliable. This faction won and they were in favor of certain cosmetic changes to the M16 to give the illusion that it was now a stronger, more robust weapon system. The "government profile" barrel is one such change, the 3/4 inch profile forward of the front sight is completely pointless, and stories of bending barrels is a myth. The fact is that section of barrel on an M16 has thicker walls than the M14 barrel forward of the stock, and no one claimed to have bent M14 barrels.
In reality, just about every change made to the M16A1E1 was of little or no value to the Army.
1. Rear Sights - The Army doesn't adjust sights except to zero, so messing around with a nail once every two years or so is not a big issue.
2. Front Sight Post - Round, or square it doesn't matter, unless your target shooting.
3. Burst fire - It was a fad at the time . . .
4. "Improved" furniture - Better plastic, better hand guard
5. Case deflector - Plus
6. Extra Stock Length - Unnecessary for the Army at the time and turned out to be a bad choice as body armor became more prevalent.
7. Minor changes to the lower to strengthen the front pivot lugs and the receiver extension bow.
8. New pistol grip
Of these changes, #4, #5 and #7 were the only useful one to the Army and could have been introduced on an attritional basis much cheaper.
_ _ _ _ _ _ _ _ _ _ _ _ _
* The M14 barrel is .595" in diameter with a .30" hole, the M16 barrel is .570" in diameter with a .22" hole, it actually has a thicker wall, but the because the outside diameter is smaller the two barrels have about the same stiffness.
BufordTJustice
10-25-23, 14:12
I haven't chimed in as Todd had covered it pretty well.
As to "Analysis of M16A2 Rifle Characteristics and Recommended Improvements" . . . . (the above quoted report)
In the mid-1980s there two factions in the Army, one felt that the M16A1 had overcome all of its teething troubles and was now a mature and reliable system, and in a few years, the poor reputation of the early 60s would be replaced as more soldiers used and saw that it was a reliable system. And to replace it with something different at this point would give the implication that the M16A1 was still deficient in reliability or durability.
The other faction believed that the M16A1 had overcome all of its teething troubles and was now a mature and reliable system, BUT, still had a "perception" problem with the troops, as they perceived it to be fragile and unreliable. This faction won and they were in favor of certain cosmetic changes to the M16 to give the illusion that it was now a stronger, more robust weapon system. The "government profile" barrel is one such change, the 3/4 inch profile forward of the front sight is completely pointless, and stories of bending barrels is a myth. The fact is that section of barrel on an M16 has thicker walls than the M14 barrel forward of the stock, and no one claimed to have bent M14 barrels.
In reality, just about every change made to the M16A1E1 was of little or no value to the Army.
1. Rear Sights - The Army doesn't adjust sights except to zero, so messing around with a nail once every two years or so is not a big issue.
2. Front Sight Post - Round, or square it doesn't matter, unless your target shooting.
3. Burst fire - It was a fad at the time . . .
4. "Improved" furniture - Better plastic, better hand guard
5. Case deflector - Plus
6. Extra Stock Length - Unnecessary for the Army at the time and turned out to be a bad choice as body armor became more prevalent.
7. Minor changes to the lower to strengthen the front pivot lugs and the receiver extension bow.
8. New pistol grip
Of these changes, #4, #5 and #7 were the only useful one to the Army and could have been introduced on an attritional basis much cheaper.
_ _ _ _ _ _ _ _ _ _ _ _ _
* The M14 barrel is .595" in diameter with a .30" hole, the M16 barrel is .570" in diameter with a .22" hole, it actually has a thicker wall, but the because the outside diameter is smaller the two barrels have about the same stiffness.
My first patrol carbine was a 1033 program A1. Well worn, but the trigger was fantastic, it had nill recoil with even the hottest loads, and using 55gr it was actually VERY accurate/precise.... sitting on the cusp of MOA with Federal M193 during agency training and quals. Just a joy to shoot.
Did I mention how incredible that well-worn FA trigger (minus the 3rd pin and other fun parts) was? It was perfect. Broke at ~4.8lbs. BUTTER smooth with a crisp break. 100% reliable. No drag. Just a joy. I say this as an unadulterated Geissele snob who loves his SSA-EX lightning bow triggers.
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My first patrol carbine was a 1033 program A1. Well worn, but the trigger was fantastic, it had nill recoil with even the hottest loads, and using 55gr it was actually VERY accurate/precise.... sitting on the cusp of MOA with Federal M193 during agency training and quals. Just a joy to shoot.
Did I mention how incredible that well-worn FA trigger (minus the 3rd pin and other fun parts) was? It was perfect. Broke at ~4.8lbs. BUTTER smooth with a crisp break. 100% reliable. No drag. Just a joy. I say this as an unadulterated Geissele snob who loves his SSA-EX lightning bow triggers.
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My agency received 400 refurbished M16A1 rifles from the DOA. I had the opportunity to fire several of them using Federal M193. They were fairly accurate out to 400 yards.
In response to Lysander's comments, I like the triangular handguard and shorter buttstock better than the furniture on the A2. If I am not shooting a High-Power Rifle match where I need to make sight changes to compensate for wind and light, I prefer the old school rear sight.
I never liked the 1:7 twist barrel. I prefer a 1:9 twist barrel for most purposes and a 1:8 twist barrel for shooting 80g projectiles when X Ring hits count at 600 yards.
vicious_cb
10-25-23, 17:17
The following is extracted from "Black Rifle II", chapter 1:
"Many experts claimed the 1 turn in 9" would have been much more desirable for many reasons, primarily longevity and durability. The much faster 1-in-7" twist caused premature wear, and was found to be significantly more likely to burn out barrels on automatic/burst fire. Additionally, the 1-in-7" twist definitely overstabilizes the 55-grain M193 ball projectile, degrading it's terminal performance."
So, almost 20 years after this book was published, is this still seen as holding true?
Did you check page 3 of your very own thread?
https://www.m4carbine.net/showthread.php?240050-The-1-8-barrel-twist
Yep, and that pretty much has to do with the material property of the steel, how hot you get your barrel and the flame temp of the powder you're burning of which there is very little data on.
Actually, there is an abundance of data on the subject, it is just that little of it is published in layman's terms, or easily accessed.
However, in simplistic terms, ball propellants are the mildest for throat erosion, then single based stick, and last double based stick.
There is a reason 90% of all military small arms (30 mm and smaller) ammunition is loaded with ball propellant, it is the easiest on barrels. There is a ton of test data showing barrels that use ball propellant exclusively out last other propellant types.
There are two reasons for this, 1) ball propellant is heavily deterred to achieve progressive burn characteristics, so it's flame temperature is a good bit lower than other propellants, and 2) the calcium carbonate left over from processing is very good at reducing throat erosion, but it does make the propellant "sooty".
There is no such thing as “over stabilization”, inconsistent FMJ terminal performance is caused by AOA and fleet yaw.
To be pedantic I believe there is such a thing, it just doesn't apply here. Or do you disagree with this passage?
Bryan Litz says:
July 25, 2013 at 7:24 am
For practical purposes, the answer is FALSE
As a bullet arcs on a long range trajectory, it’s axis is torqued (by aerodynamic forces) to constantly align with the oncoming airflow. When a spinning object has its axis torqued, the object reacts by pointing its axis primarily ‘out of plane'; 90 degrees from the applied force. This results in a nose-right orientation (for right twist barrels) known as the yaw of repose. The yaw of repose steers the bullet ever so slightly to the side resulting in gyroscopic (spin) drift.
The bullet nose will point slightly above its velocity vector (pitch), but that pitch is only about 1/10 of the yaw of repose which is not enough to cause a practical vertical drift (less than 1/2″ at 1000 yards). Typical yaw of repose remains below 1/60th of one degree, while pitch is on the order of 1/600th of one degree. This small amount of pitch and yaw is not enough to induce a measurable amount of additional drag, even for highly stabilized bullets.
All of the above applies to stable projectiles in supersonic flight on ‘flat fire’ trajectories. For projectiles fired at high angles (above ~10-20 degrees above the line of sight), it is possible for the bullet to not track, or trace with the trajectory. This is a common design challenge for artillery shells that are often fired on high angle trajectories. The axis of the spinning shell may be too rigid to bend with the exaggerated trajectory. In that case the shell can ‘belly flop’, or fall base first. However for small arms projectiles on flat-fire trajectories, this isn’t a problem.
Another consideration with spinning a bullet too fast is related to bullet failure. This discussion assumes the bullet remains structurally in tact.
Dynamic instability during transonic flight is also a different problem, not related to the above discussion.
I don't think you can "overstabilize" anything. It's either stable or it's not. I think you can spin a projectile too fast for its performance parameters.
I don't think you can "overstabilize" anything. It's either stable or it's not. I think you can spin a projectile too fast for its performance parameters.
What term would you use to describe the condition on the bold passage?
What term would you use to describe the condition on the bold passage?
"Overspinning" or "spinning beyond performance parameters" if the rotation of the projectile is too fast for optimum performance in flight.
I love the "twist rates affect terminal performance" discussions. We've been having them for decades, right back to the point where they switched from 1/14 barrels to 1/12, and people started complaining that M193 no longer had the horrific wounding potential it used to have. Keep moving forward, and the same argument is had with the 1/12 to 1/7 switch. Maybe perception is the problem, as well as the fact that M193 likely was never as good as its legend would indicate.
To be pedantic I believe there is such a thing, it just doesn't apply here. Or do you disagree with this passage?
“However for small arms projectiles on flat-fire trajectories, this isn’t a problem.”
I agree with this. Artillery does have some concerns but it’s not relevant to the issue here.
To be clear, my comment is specifically about terminal performance in the M16 family and the myth of “over stabilization”. My comments are generally limited to the practical or common range of twists, bullet weights, and use of a rifle.
BufordTJustice
10-26-23, 12:19
I love the "twist rates affect terminal performance" discussions. We've been having them for decades, right back to the point where they switched from 1/14 barrels to 1/12, and people started complaining that M193 no longer had the horrific wounding potential it used to have. Keep moving forward, and the same argument is had with the 1/12 to 1/7 switch. Maybe perception is the problem, as well as the fact that M193 likely was never as good as its legend would indicate.
My instinct is to agree with everything you've said.
But, when I think about what sold the entire defense establishment (or the heads and necks of it) on this .22 caliber round over the 7.62 Nato's M80 ball, it makes me wonder if there may be some truth to 1/14 doing some really crazy sh**, terminally.
Really wish hard data existed regarding this. I am not aware of a 1/14 twist barrel available today that could be used to test this hypothesis.
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The early reports from DARPA regarding the AR-15's performance in Vietnam are impressive, but some have accused them of being embellished to some extent.
There is hard data. Look up AOA and fleet yaw.
In a basic overview, both sides were right. Not about the twist, but both good and poor terminal performance.
Sometimes the bullet hits the target with enough yaw to tumble, fragment, and leave an impressive wound. Sometimes it pokes a small hole straight through without doing as much damage.
I believe this is the report that is frequently cited in the discussions of early AR-15 combat effectiveness:
(2.) (C) "On 9 June a Ranger Platoon from the 40th nf Regt was given the mission of ambushing an estimated VC Company. The details are as follows:
a. Number of VC killed: 5
b. Number of AR-15's employed: 5
c. Range of engagement: 30-100 meters
d. Type wounds:
1. Back wound, which caused the thoracic cavity to explode.
2. Stomach wound, which caused the abdominal cavity to explode.
3. Buttock wound, which destroyed all tissue of both buttocks.
4. Chest wound from right to left, destroyed the thoracic cavity.
5. Heel wound, the projectile entered the bottom of the right foot causing the leg to split from the foot to the hip.
These deaths were inflicted by the AR-IS and all were instantaneous except the buttock wound. He lived approximately five minutes.
https://apps.dtic.mil/sti/pdfs/AD0343778.pdf
BufordTJustice
10-26-23, 13:39
There is hard data. Look up AOA and fleet yaw.
In a basic overview, both sides were right. Not about the twist, but both good and poor terminal performance.
Sometimes the bullet hits the target with enough yaw to tumble, fragment, and leave an impressive wound. Sometimes it pokes a small hole straight through without doing as much damage.
Very familiar with AOA and Fleet Yaw. [emoji106]
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My instinct is to agree with everything you've said.
But, when I think about what sold the entire defense establishment (or the heads and necks of it) on this .22 caliber round over the 7.62 Nato's M80 ball, it makes me wonder if there may be some truth to 1/14 doing some really crazy sh**, terminally.
Really wish hard data existed regarding this. I am not aware of a 1/14 twist barrel available today that could be used to test this hypothesis.
Yeah, I can't say I've ever seen a 1/14 barrel new on the market. They probably exist, but even the Brownells Retro guns like the AR-15 prototype used 1/12 rather than 1/14.
The early reports from DARPA regarding the AR-15's performance in Vietnam are impressive, but some have accused them of being embellished to some extent.
Bingo. My personal opinion is that McNamara wanted the military to move on from the M14 so he could cut squad size and manpower requirements, and anything that supported that goal was pushed (such as adopting the new wonder gun, the AR-15). As with any statistic, when you go looking for data to support a preselected conclusion, you likely will find it. This isn't to say that the incidents are false or never happened; they likely were relatively close to the fact. They just also were likely the upper end of capability and not the average as was frequently implied.
Yeah, I can't say I've ever seen a 1/14 barrel new on the market. They probably exist, but even the Brownells Retro guns like the AR-15 prototype used 1/12 rather than 1/14.
Green Mountain had a 1/14 a few years back, but not sure if they will again.
I'll add another one to your original question.......Why are there an increasing number of 1:8 on the market today, when they never used to be offered?
1/8 has been around since 1995 or so in CMP/DCM rifles. Seems like there were even options for 1/7.xx and1/8.xx twists too.
The following is extracted from "Black Rifle II", chapter 1:
"Many experts claimed the 1 turn in 9" would have been much more desirable for many reasons, primarily longevity and durability. The much faster 1-in-7" twist caused premature wear, and was found to be significantly more likely to burn out barrels on automatic/burst fire. Additionally, the 1-in-7" twist definitely overstabilizes the 55-grain M193 ball projectile, degrading it's terminal performance."
The portion in red is flat out false, so what other parts of the statement are actually true?
....
Why are there an increasing number of 1:8 on the market today, when they never used to be offered? I shoot primarily 77gr reloads so I opt for the faster twists.....as per what appears to still be consensus.
.
1:8" twist barrels have been in use for ages by Service Rifle shooters and accuracy aficionados. The 1:8" twist works great with 77 grain OTM bullets. The 10-shot group pictured below was fired prone off a bi-pod from my Lothar Walther barreled AR-15. The group has an extreme spread of 0.54 MOA.
https://www.ar15.com/media/mediaFiles/28568/lothar_walther_77_smk_bipod_100_yards_me-3007634.jpg
....
To be pedantic I believe there is such a thing,
Testing performed by C.E. Harris at Aberdeen Proving Ground and later at Sturm-Ruger has shown that the “overstabilization” claim is largely nonsense. The testing showed that “overspinning” quality light-weight bullets from a fast twist barrel does not become an issue unless you have a gyroscopic stability factor greater than 5.0 (which would require something along the lines of a 1:6” twist barrel launching a 55 grain bullet at over 3500 fps) or unless firing at an angle greater than 85 degrees.
After the U.S. Military adopted the 1:7” twist for the M16A2, C.E. Harris did extensive testing comparing the accuracy of light-weight bullets fired from 1:10” twist barrels and 1:7” twist barrels using 52 grain Sierra MatchKings. The accuracy testing was done from 200 yards, (well into the downward slope of the trajectory where the Internet Commando claims that all manner of evil befalls the “overstabilzied” bullet), and the accuracy results from the two different twist barrels were also nearly identical.
By definition, an “unstable” bullet will have a gyroscopic stability factor of less than 1.0 at the muzzle. A typical 55 grain FMJ bullet will have a gyroscopic stability factor of approximately 4.27 when fired from a 20” barrel with a 1:7” twist.
4.27 is not less than 1.0.
The following demonstration compares the results of firing four 10-shot groups of the same lot of 55 grain Prvi Partizan M193 ammunition from two different barrels; one barrel with a 1:9” twist, the other barrel with a 1:7” twist.
The first barrel used in testing was 16” Colt HBAR with chrome lining, a NATO chamber and a 1:9” twist. This is the barrel found on the Colt 6721. All of my free-floated Colt 6721 barrels have turned in sub-MOA 10-shot groups at 100 yards when using match-grade hand-loads.
The second barrel used in testing was a 20” Colt HBAR, also with chrome-lining, a NATO chamber and of course a 1:7” twist. I've owned three of these barrels and they have all turned in 10-shot groups at 100 yards that hover just above one MOA when free-floated and shooting match grade handloads. The longer barrel with the 1:7” twist was purposely chosen for the increased muzzle velocity coupled with the 1:7” twist.
Accuracy (technically, precision) testing was conducted from a distance of 100 yards following my usual protocol. The barrels were free-floated during testing. The fore-ends of the weapons rested in a Sinclair Windage Benchrest and the butt-stock rode in a Protektor rear-bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Naturally, the wind conditions were monitored using a Wind Probe.
Four 10-shot groups of the PPU M193 were fired from the 1:9” twist barrel. Those groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius of that composite group was 1.08”.
As with the 1:9” twist, four 10-shot groups were fired from the 1:7” twist barrel. Those groups were also also over-layed on each other to form a 40-shot composite group; the results were nearly identical to those obtained from the 1:9” twist barrel. The composite group had a mean radius of 1.01”.
The entire test as described above was also conducted using a second 16" chrome-lined, NATO chambered Colt HBAR with a 1:9" twist and a second 20" chrome-lined, NATO chambered Colt HBAR with a 1:7" twist. The ammunition used in this test was all from the same lot of Wolf 55 grain FMJ "Performance Ammunition."
As before, four 10-shot groups fired from each barrel at 100 yards were over-layed on each other using RSI Shooting and just as before, the mean radii for these 40-shot composite groups were nearly identical.
t makes me wonder if there may be some truth to 1/14 doing some really crazy sh**, terminally. Really wish hard data existed regarding this.
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Hard data does exist, but the US military didn't release it to the general public, but Dr GK Roberts has mentioned it before . . .
From Dr. G.K. Roberts . . .
"The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists. Similar testing was conducted with 5.56 mm 62 gr M855 FMJ ammunition fired in 1/9 and 1/7 twist barrels. Again, no difference in terminal performance was noted."
Seems like there were even options for 1/7.xx and1/8.xx twists too.
I've owned multiple Krieger AR-15 barrels with 1:7.7" twists.
https://i.ibb.co/7VnPQmr/krieger-barrel-stamp-023.jpg
....
BufordTJustice
10-28-23, 19:19
Hard data does exist, but the US military didn't release it to the general public, but Dr GK Roberts has mentioned it before . . .
From Dr. G.K. Roberts . . .
"The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists. Similar testing was conducted with 5.56 mm 62 gr M855 FMJ ammunition fired in 1/9 and 1/7 twist barrels. Again, no difference in terminal performance was noted."
Thanks Molon. Good to see a DocGKR quote. [emoji106][emoji106]
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Testing performed by C.E. Harris at Aberdeen Proving Ground and later at Sturm-Ruger has shown that the “overstabilization” claim is largely nonsense.
Since you quoted me in preamble to this statement I think you must have missed the quote in my own post. It may be nonsense in the domain of rifles and terminal ballistics while still being a real effect in the domain of artillery.
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