The 1:8 barrel twist

[constructor]

Seems like higher quality barrels use 1:7.7 or 1:8. When we were testing barrels in 2007 most of the 1:7 twist 6 groove barrels did not meet the minimum bore area spec. that causes higher pressures and more engraving force.

[Molon]

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.

The Hornady 70 grain GMX has a nominal length of 1.045”. Per Ballistic Performance of Rifle Bullets, the 70 grain GMX has a nominal gyroscopic stability factor of 1.37 when fired from a barrel with a 1:8” twist. The factory loaded 5.56mm 70 grain GMX shot well from my 1:8” twist Lothar Walther barreled AR-15.





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[markm]

Seems like higher quality barrels use 1:7.7 or 1:8. When we were testing barrels in 2007 most of the 1:7 twist 6 groove barrels did not meet the minimum bore area spec. that causes higher pressures and more engraving force.

We run a 30 cal cut rifle barrel made a thowy or so under-bored. It's chambered in 300WM and definitely bucks a bit harder.

[Slater]

I occasionally read opinions that, all else being equal, a 1/9 barrel will have a longer service life than a 1/7 barrel. Any validity to that?

[constructor]

We run a 30 cal cut rifle barrel made a thowy or so under-bored. It's chambered in 300WM and definitely bucks a bit harder.

I have had a small bore Krieger 300RSAUM Palma contour barrel since 2008, one of the most accurate barrels I have ever owned. It's a perfect match for Lapua bullets.

[georgeib]

The Hornady 70 grain GMX has a nominal length of 1.045”. Per Ballistic Performance of Rifle Bullets, the 70 grain GMX has a nominal gyroscopic stability factor of 1.37 when fired from a barrel with a 1:8” twist. The factory loaded 5.56mm 70 grain GMX shot well from my 1:8” twist Lothar Walther barreled AR-15.





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Thank you for that correction!

[DG23]

I occasionally read opinions that, all else being equal, a 1/9 barrel will have a longer service life than a 1/7 barrel. Any validity to that?

Going to cost me a pile of money and time to wear out one if I have all three to shoot from...

1/7, 1/8, 1/9

[markm]

I occasionally read opinions that, all else being equal, a 1/9 barrel will have a longer service life than a 1/7 barrel. Any validity to that?

I remember that notion when ARs started to evolve from the old A1 1/12 to 1/9 and finally to 1/7. The reality is that throat erosion is the barrel killer, and twist rate has no impact on that wear.

[vicious_cb]

I remember that notion when ARs started to evolve from the old A1 1/12 to 1/9 and finally to 1/7. The reality is that throat erosion is the barrel killer, and twist rate has no impact on that wear.

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.

[lysander]

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.