The 1:8 barrel twist

[MSW]

FWIW, my Compass Lake built SAM-R clone is a 20” Krieger 1/7.7 twist, CLE chamber. The build record notes the twist. CLE now use 1/7, but when I spoke with the elder owner around 2016, he said the 1/7.7 gave the best accuracy with the Mk262 ammo. Or, maybe he had a bunch of unused 1/7.7 barrels he needed to sell….

Grain of salt time?

[Wake27]

BCMs KD4 barrels are 1/7.7 and optimized for MK262 as well.


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[498cm3]

^^ Yes. I think Criterion does too.

Come to think of it, I believe Fulton gets most of their barrels from them.

[SteveL]

SOLGW match barrels are also 1:8.

BCMs KD4 barrels are 1/7.7 and optimized for MK262 as well.


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When I took a scoped rifle class with him a few years back these uppers had just hit the market. He talked about them in the class and about how 1:7.7 performed better with MK262 than 1:7 or 1:8.

[Molon]

It’s largely Internet Commando myth that bullet stability increases with increasing barrel length. Technically, the gyroscopic stability factor does increase with increased barrel length, however the amount that it increases is miniscule and lost in the noise of other variables.

As an example, the 55 grain bullet in M193 will have a gyroscopic stability factor of approximately 4.25 when fired from a 14.5” Colt M4 barrel. When the same M193 round is fired from the 5.75” longer barrel of the 20” Colt A2 barrel, the gyroscopic stability factor of the 55 grain bullet only increases to approximately 4.27.

While the increased RPM due to the faster velocity will act to increase the bullet stability, the increased velocity also “increases the force applied to the nose of the bullet at the center of pressure and strengthens the overturning aerodynamic torque which actually makes the bullet less stable.”* These two opposing dynamics are the reason for the miniscule increase in gyroscopic stability factor.


* From Applied Ballistcs For Long-Range Shooting by Bryan Litz

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[Molon]

SOLGW match barrels are also 1:8. When I took a scoped rifle class with him a few years back these uppers had just hit the market. He talked about them in the class and about how 1:7.7 performed better with MK262 than 1:7 or 1:8.

I'm sure he provided you with statistically significant shot-group data for MK262 fired from AR-15s with premium barrels of varying twist rates to support his claim, right? Per Ballistic Performance of Rifle Bullets, the "recommended twist for optimal performance is 1:8.7" or faster" for the 77 grain SMK. When fired from a barrel with a 1:8" twist, the 77 grain SMK has a nominal gyroscopic stability factor of 1.75.

The 10-shot group pictured below was fired from an AR-15 with a 1:8" twist Lothar Walther barrel at 100 yards using Black Hills MK262. The group has an extreme spread of 0.96".






A 10-shot group of hand-loads topped with the 77 grain SMK fired from the same 1:8" twist AR-15 had an extreme spread 0.56".







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[vicious_cb]

There's not a ton of AR's on the market with this particular twist, but I see a few every now and then. What advantage (if any) is there in the 1:8?

Zip, zero, nada, none over a 1/7. Barrel quality matters far more so just pick the best one you can afford. In fact I prefer the faster twist rate when you see how long the top of line performing ammo like 77gr TMK, Brown Tip ect are.

[georgeib]

As a data point I'll add my 2 cents. I only have a single 1:8 barrel in 223 Wylde. It came in a 16" PWS rifle that I have. According to Strelok Pro, the Barnes 70gr VOR-TX has a gyroscopic stability factor of only 1.33. The Barnes 70gr. VOR-TX bullets are solid copper and have a nominal length of 1.073", so I'm sure that's a factor. As it so happens that particular load out of my particular rifle groups like shit.

I can't remember the exact numbers, but I want to say it was around 1-1.5" at 25 yards. Badly enough that I wouldn't want to shoot any of it through a suppressor on that particular rifle, and I didn't bother with further distances. The same rifle will shoot 5 rounds of IMI Razorcore into a ragged hole at 25, and around an inch at 100. Heck, it groups m193 better than it does the 70gr Barnes.

ETA: I remembered that this barrel didn't do great with Black Hills 70gr GMX Barrier either. That ammo uses Hornady 70gr GMX all copper bullets with a nominal length of 0.9" and produces a gyroscopic stability factor of 2.19 according to Strelok Pro. Not as bad as the Barnes, as I recall, but not good by any stretch.

This has put me on the hunt to build a more precision orientated upper, but is shying me away from anything slower than 1:7. Which is posing some difficulty as far as available options.

[markm]

Sionics uses 1:8.

The Sionics on my lap is marked 1/7. Perhaps current production is different. I heard they also no longer offer the FSB. That sucks.

It’s largely Internet Commando myth that bullet stability increases with increasing barrel length. Technically, the gyroscopic stability factor does increase with increased barrel length, however the amount that it increases is miniscule and lost in the noise of other variables.

I did an experiment where I had some relatively long (for the twist) bullets. And although barrel length was the same, the increase in velocity (and thus bullet spin) took the bullet from key holing to stability and modest grouping.

I honestly did not expect the additional velocity to do anything for stability, but I was wrong.

[Molon]

As a data point I'll add my 2 cents. I only have a single 1:8 barrel in 223 Wylde. It came in a 16" PWS rifle that I have. According to Strelok Pro, the Barnes 70gr VOR-TX has a gyroscopic stability factor of only 1.33. The Barnes 70gr. VOR-TX bullets are solid copper and have a nominal length of 1.073", so I'm sure that's a factor. As it so happens that particular load out of my particular rifle groups like shit.

I can't remember the exact numbers, but I want to say it was around 1-1.5" at 25 yards. Badly enough that I wouldn't want to shoot any of it through a suppressor on that particular rifle, and I didn't bother with further distances. The same rifle will shoot 5 rounds of IMI Razorcore into a ragged hole at 25, and around an inch at 100. Heck, it groups m193 better than it does the 70gr Barnes..

The 70 grain TSX is a looong bullet. Depending upon variables, the performance of this bullet can sometimes be rather iffy from a 1:8” twist barrel.







When properly stabilized, the 70 grain TSX is capable of decent accuracy/precision. The 10-shot group pictured below was fired from an AR-15 with a 1:7.7” twist barrel at a distance of 100 yards. The group has an extreme spread of 1.12”













A bullet that is not properly stabilized can still show decent accuracy at shorter distances. The 10-shot group of the 70 grain TSX pictured below was fired from a Colt 6721 at a distance of 50 yards. The 6721 barrel has a 1:9” twist and it produced a tidy little group at 50 yards with the 70 grain TSX.






However, when the 70 grain TSX is fired from the same barrel at a distance of 100 yards, accuracy is degraded and multiple shots show key-holing.





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