1/7, 1/8, 1/9 which for a combat carbine?
- Thread starter Matt Landfair
- Start date
I posted the quote for historical reference as much as anything. The study was published in Feb '86, and is titled "Analysis of the M16A2 rifle characteristics and recommended improvements " looks like a joint effort between Litton Systems and the Army Research Institute. There are numerous references in the bibliography. The study states the independent testing conducted by the NRA showed the A1 firing M193 was more accurate at 200mtrs then the A2 firing M855. They also site AMU practices at the time.
It looks like the Army wasn't too enamored with M855 to start with, and wasn't all that interested in performance much beyond 300mtrs. The M16 is afterall, an assault rifle.
I don't know what voodoo, bullet and cartridge designers use or used in the past to determine what bullet weight is optimal for the cartridge they are designing, or where 55grn for M193 came from. I'm fairly sure there are boatloads of dead bodies with 55grn projectiles in them. To quote a former co-worker and 173Abn vet, 1968..."we killed so many of them , we needed bulldozers to bury the bodies".
Where did 77grns come from? Was it an outgrowth of the 1/7 twist that was already common? that was in turn an out growth of the tracer round?
So what comes first?
Bullet weight/design?
Rifling twist?
Desired Max effective range?
Last time I looked, and it's been awhile, Doc had acceptable loads for most twist rates, and BH has a 50grn bullet that was liked. Though I think it's longer then most..
Had the Army gone with 1/9 twist, would we see more bullet's designed for that twist rate? and possibly not even be thinking about 1/7?
The study stated increased barrel life and reduced fouling as a benefit to the 1/9...is that not the case, or overstated?
Also, they noted they wanted to test alternatives to 1/7, and were using 1/9 as a comparison, not a recommendation.
Right now, I think all my barrels are 1/7, just because that's what most quality carbine type barrels come in. I have a Criterion hybrid that might be 1/8, and I think the Satern I ordered was 1/8....but I haven't seen that barrel in years...
FWIW: The study has a lot of interesting tid bits in it. It looks like a lot of what the Army wanted at the time, the A2 didn't provide, and not until the adoption of the M4 did they get some of it. Interesting reading.
It looks like the Army wasn't too enamored with M855 to start with, and wasn't all that interested in performance much beyond 300mtrs. The M16 is afterall, an assault rifle.
I don't know what voodoo, bullet and cartridge designers use or used in the past to determine what bullet weight is optimal for the cartridge they are designing, or where 55grn for M193 came from. I'm fairly sure there are boatloads of dead bodies with 55grn projectiles in them. To quote a former co-worker and 173Abn vet, 1968..."we killed so many of them , we needed bulldozers to bury the bodies".
Where did 77grns come from? Was it an outgrowth of the 1/7 twist that was already common? that was in turn an out growth of the tracer round?
So what comes first?
Bullet weight/design?
Rifling twist?
Desired Max effective range?
Last time I looked, and it's been awhile, Doc had acceptable loads for most twist rates, and BH has a 50grn bullet that was liked. Though I think it's longer then most..
Had the Army gone with 1/9 twist, would we see more bullet's designed for that twist rate? and possibly not even be thinking about 1/7?
The study stated increased barrel life and reduced fouling as a benefit to the 1/9...is that not the case, or overstated?
Also, they noted they wanted to test alternatives to 1/7, and were using 1/9 as a comparison, not a recommendation.
Right now, I think all my barrels are 1/7, just because that's what most quality carbine type barrels come in. I have a Criterion hybrid that might be 1/8, and I think the Satern I ordered was 1/8....but I haven't seen that barrel in years...
FWIW: The study has a lot of interesting tid bits in it. It looks like a lot of what the Army wanted at the time, the A2 didn't provide, and not until the adoption of the M4 did they get some of it. Interesting reading.
Caomhin
Member
American Rifleman magazine had an article within the last six or seven months claimed that 1:8 decreased group sizes of M855 (62gr) in half.
I've not tried to verify it but even if it's a mildly exaggerated claim it's worthy of study.
If I recall correctly, and I so NOT, I'm not sure if the 1:8 twist worked as well with tracers.
I dunno.
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"One cannot awaken a man who pretends to be asleep."
I've not tried to verify it but even if it's a mildly exaggerated claim it's worthy of study.
If I recall correctly, and I so NOT, I'm not sure if the 1:8 twist worked as well with tracers.
I dunno.
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"One cannot awaken a man who pretends to be asleep."
Practically Tactical
Newbie
I started dabbling into this voodoo when I saw Green Mountain was making 1:6 barrels for testing. From what I read was that over stabilization would happen in lighter bullets, specifically it would basically make the tip of the bullet angle up and not hit tip first. I hope I explained that right..
I'm assuming the M249 has a 1/7 twist and is what FN determined was needed for the tracer round when they developed the gun and the SS109.
The study mentions the the marines went with 1/7 to stabilize the tracer at extended ranges, but felt that wasn't an issue, as tracers fired in the rifle are used to mark targets at extended ranges and precise accuracy is not required.
Just some interesting history to think on, is all
The study mentions the the marines went with 1/7 to stabilize the tracer at extended ranges, but felt that wasn't an issue, as tracers fired in the rifle are used to mark targets at extended ranges and precise accuracy is not required.
Just some interesting history to think on, is all
Turkishroy
Newbie
@rob_s it is a matter of length. My advanced ballistics book, IIRC, states the most stable projectile of a given caliber is one that is five lengths it's caliber. That's usually how you end up with the most accurate rounds in a cartridge as the heaviest ones, though it's driven by gyroscopic stability, not the need for a particularly heavy round.
From personal experience in hand loading, the empirical difference inside of 300 is difficult to even see under less than ideal situations ( plenty of time to shoot, stable position, experienced shooter, quality weapon.)
With the same 1/7 Daniel Defense 18" barrel I'll group carefully loaded 75gr BTHPs and mk262 clones in about a half inch at 100yds and 52gr BTHPs just over an inch.
Outside of precision concerns, I'd stick with heavier rounds to lessen wind effects. A full value cross wind will mean more to a lighter round than it will a heavier one, though it usually takes more distance to see the effect on paper.
If shooting in less than sterile conditions, you'll have less need to concern yourself with wind conditions when shooting at extended ranges.
From personal experience in hand loading, the empirical difference inside of 300 is difficult to even see under less than ideal situations ( plenty of time to shoot, stable position, experienced shooter, quality weapon.)
With the same 1/7 Daniel Defense 18" barrel I'll group carefully loaded 75gr BTHPs and mk262 clones in about a half inch at 100yds and 52gr BTHPs just over an inch.
Outside of precision concerns, I'd stick with heavier rounds to lessen wind effects. A full value cross wind will mean more to a lighter round than it will a heavier one, though it usually takes more distance to see the effect on paper.
If shooting in less than sterile conditions, you'll have less need to concern yourself with wind conditions when shooting at extended ranges.
Alrighty- I'll dive in as this is an area of were my knowledge base bleeds over.
The big two caveat one has to establish in all of this when talking about the relationship between a projectile and the twist rate its being launched out of is that it is both a trend based, and bullet subjective. I'll lay them out in bullet point to save some time.
-Faster twist rates do lower a projo's muzzle velocity, but by such a small measure that its not worth even worrying about. On the testing done by Bryan Litz, the lost velocity was I think on average 12 FPS.
-Faster twist rates do raise the bullet's ballistic coefficient by increasing the stability of the projectile, how much is subjective to the bullet's weight, sectional density, and its dimensions. This has the side benefit of keeping a bullet stable as it approach's and surpasses transonic, as well as better overall muzzle velocity retention due the increase in BC.
-Faster twist rates do result in larger group sizes by virtue of the increases gyroscopic force inflicted on the projectile. How much is again subject to the unique characteristics of the bullet fired. This has a lot to do with how well the lead core is placed inside the copper jacket for obvious reasons. Match grade bullets are obviously going to be better then run of the mil production stuff since they are made with more consistent methods and stringent quality control.
Personally, I see more benefit then anything. That state of the art in long range shooting is leaning toward's faster twist rates, specifically as bullet shapes get longer and grain weights get heavier. 20-30 years ago we couldn't imagine pushing some of the bullets with the BC's currently available... but thats all changing.
For those wishing to play with and find an optimal twist rates for their projectiles, Berger has a pretty solid calculator on their website for calculating the stability of a specific projectile relative to their twist rate, as well as the associated bump in BC.
http://www.bergerbullets.com/twist-rate-calculator/
The big two caveat one has to establish in all of this when talking about the relationship between a projectile and the twist rate its being launched out of is that it is both a trend based, and bullet subjective. I'll lay them out in bullet point to save some time.
-Faster twist rates do lower a projo's muzzle velocity, but by such a small measure that its not worth even worrying about. On the testing done by Bryan Litz, the lost velocity was I think on average 12 FPS.
-Faster twist rates do raise the bullet's ballistic coefficient by increasing the stability of the projectile, how much is subjective to the bullet's weight, sectional density, and its dimensions. This has the side benefit of keeping a bullet stable as it approach's and surpasses transonic, as well as better overall muzzle velocity retention due the increase in BC.
-Faster twist rates do result in larger group sizes by virtue of the increases gyroscopic force inflicted on the projectile. How much is again subject to the unique characteristics of the bullet fired. This has a lot to do with how well the lead core is placed inside the copper jacket for obvious reasons. Match grade bullets are obviously going to be better then run of the mil production stuff since they are made with more consistent methods and stringent quality control.
Personally, I see more benefit then anything. That state of the art in long range shooting is leaning toward's faster twist rates, specifically as bullet shapes get longer and grain weights get heavier. 20-30 years ago we couldn't imagine pushing some of the bullets with the BC's currently available... but thats all changing.
For those wishing to play with and find an optimal twist rates for their projectiles, Berger has a pretty solid calculator on their website for calculating the stability of a specific projectile relative to their twist rate, as well as the associated bump in BC.
http://www.bergerbullets.com/twist-rate-calculator/
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Caomhin
Member
I've been told by others who have much more experience than my theoretical knowledge on this subject the exact same thing.
That would be my recommendation if it could be had (ex: cold hammer forged, chrome plated).
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"One cannot awaken a man who pretends to be asleep."
The rub with magazine fed weapons is that your limited to the COAL that the magazine allows. So realistically your only going to get so heavy and long with those projectiles, and they tend to be or of a tangent ogive shape because its forgiving to load for good accuracy without being able to fine tune the distance between ogive and land/groove's like you can on a bolt action rifle.
So logically, with magazine fed weapons this is the rub when it comes to selecting a bullet/twist rate combo.
So logically, with magazine fed weapons this is the rub when it comes to selecting a bullet/twist rate combo.
*shrug* I personally think choosing a twist rate to shoot one particular type of ammo is a waste, especially if you are sacrificing accuracy at distance, and overall portability of the platform becuase you have to run a 20 inch barrel to make it work. You can get some pretty legit wounds with tight twist rates in 14.5-16 inch in the heavier grain weights.
medic15al
Newbie
I based my preference on that above and shooting and finding 55 grain as well as 50 grain ammo much easier in my area. I already have an M-16A1 clone with 1:12 barrel so I may as well use that to my advantage. I find the 60 grain Nosler Partition shoots very well in this barrel as well, 1.5 in. average with a scope at 100yds. I do have a 1:7 rifle as well that likes the 69 and 77grain OTM bullets, but most of my shooting is with 55 grain loads.
This.
55 Grain Bullets Fired From AR-15s with 1:7" Twist Barrels
"55 grain bullets are unstable, over-stabilized, inaccurate, when fired from an AR-15 with a 1:7” twist barrel."
Statements such as the one above always seemed to be proclaimed without the posting of any valid data to support that these effects occur, or that if they do occur, they do so to any degree that has any significant effect on the accuracy/precision spectrum involved with AR-15s firing M193 type ammunition.
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. [CaptainObvious] 4.27 is not less than 1.0. [/CaptainObvious]
The following demonstration compares the results of firing four 10-shot groups of the same lot of 55 grain Prvi Partizan M193 amunition 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. Wind conditions were monitored using a Wind Probe. The set-up was very similar to that pictured below.
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 two composite groups are shown side by side for comparison.
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.
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.
Quality, modern lightweight bullets of copper-jacket/lead-core construction can shoot superbly from AR-15s with fast twist barrels. Typical American manufactured 55 grain FMJ bullets do not fall into the quality category.
The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist using 55 grain BlitzKings.
The next 10-shot group pictured was fired from a Krieger barreled AR-15 with a 1:7.7” twist using 55 grain BlitzKings.
The group pictured below was fired from one of my Krieger barreled AR-15s with a 1:7.7" twist from a distance of 100 yards using Berger 55 grain HP bullets. The group has an extreme spread of 0.76".
The next group was fired from my 1:7.7" twist Krieger barreled AR-15 using factory loaded Hornady American Gunner 55 grain HP ammunition.
A 14.5" Colt M4A1 SOCOM barrel, with its NATO chamber, chrome lining and 1:7" twist can shoot quality 55 grain bullets quite well.
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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. There is ongoing new research indicating that with some projectiles and especially in shorter barrels, FASTER twists produce improved terminal ballistic performance. Also, if the bullet is not adequately stabilized in flight, then alterations in the wound profile will be evident.
Twist rate can definitely effect external ballistics. For example, in testing ammunition at the CHP Academy in the mid 1990’s, a number of lightweight, thin-jacketed, relatively high velocity .223 varmint loads were observed to disintegrate in mid-air a few yards from the muzzle when fired from fast 1/7 twist weapons, but not in slower twists; the Federal 40 gr Blitz loading was particularly problematic in this regard. Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.
Twist rate can definitely effect external ballistics. For example, in testing ammunition at the CHP Academy in the mid 1990’s, a number of lightweight, thin-jacketed, relatively high velocity .223 varmint loads were observed to disintegrate in mid-air a few yards from the muzzle when fired from fast 1/7 twist weapons, but not in slower twists; the Federal 40 gr Blitz loading was particularly problematic in this regard. Likewise, long 70+ gr projectiles don't always stabilize in 1/9 or slower twist barrels.
Longeye
Established
I don't have the lab results that Molon and DocGKR do.
However, field postmortem examination of small and medium game has consistently shown more damage with American Eagle 55 gr FMJ out of 1/7 twist LMT 14.5" relative to same load out of a 1/9 twist Colt HBAR 16. Majority of the kills were made between 55-90 yards with one outlier at 176 yards. All were one shot reductions. Over a hundred data points accured.
These results were enough to convince me of the superiority of the 1/7, particularly for someone who didn't have control over his ammo type and may be forced to use 55 FMJ. Modern expanding ammo allows alot of the disadvantages of the 1/9 to be swept under the rug. But the 1/7 is definitely more versatile in my experience.
However, field postmortem examination of small and medium game has consistently shown more damage with American Eagle 55 gr FMJ out of 1/7 twist LMT 14.5" relative to same load out of a 1/9 twist Colt HBAR 16. Majority of the kills were made between 55-90 yards with one outlier at 176 yards. All were one shot reductions. Over a hundred data points accured.
These results were enough to convince me of the superiority of the 1/7, particularly for someone who didn't have control over his ammo type and may be forced to use 55 FMJ. Modern expanding ammo allows alot of the disadvantages of the 1/9 to be swept under the rug. But the 1/7 is definitely more versatile in my experience.
