Twist Rate

[MasCasa]

Hi all. Based on your knowledge and experiences, is there a notable difference in precision between the 1:7.5" and 1:7" twist rates when shooting bullets of 100 or more grains? 

As I understand it, 6 ARC was designed with a 1:7.5" twist, but I see at least Stag and Aero Precision twist rates are 1:7.

[r.tenorio671]

...TBH, if you plan to only use projectiles/ammo in the 100gn and above weights, either twist rate will suffice. The 1:7 will provide more flexibility with the 105+ class projectiles. Keep in mind that many of the bullet manufacturers provide a recommend twist rate for their product, but it should also be acknowledged that a particular range of FPS is associated with that twist rate to achieve the BC rating of the projectile.

[StoneHendge]

I would take a look at what bullets you expect to be shooting and what velocities you expect out of whatever length barrel you choose and run them through the Berger Twist calculator at your coolest expected temperatures. If it works at 7.5, I'd go that route. No one ever talks about it and there probably has been no real research done outside of the military (who isn't going to share results), but faster twist has it's downsides with potential jacket deformation and even slippage in the barrel. Even the type of rifling and amount of grooves can come into play with the amount of centrifugal force on the jacket near the end of the barrel. As the envelope keeps getting pushed with higher and higher bc bullets, I think it gets into a damned if you do damned if you don't scenario (see 224. Valkyrie). The Proof's are 7.5 and they were the barrels used to develop the cartridge with the military. If any one has the money to test the effect of twist rate on dispersion at long range and recover fired bullets to examine them for deformation, it's your tax dollars!

I'll add that I don't think I've ever seen a 7 twist 6 Creedmoor.

[Dino11]

I believe that the jacket on bullets made today have come a long way. They are much better than bullets of yesteryear. They are sometimes thicker, and the metallurgy has come a long way. It also depends on the quality of the bullets you are loading.

I believe Berger has a write-up on this in their tutorials. I don't think that the 6 ARC has the capabilities to push bullets to the ragged edge, I think you would have to see velocities above 3200 FPS.

[arcfan]

The Berger twist rate calculator: https://bergerbullets.com/twist-rate-calculator/

[BluntForceTrauma]

A 1:7 twist rate is needed, from a technical perspective, to stabilize the heavy 6mm bullets in short barrels like a 12". For longer barrels, it's not technically "needed," but as Brian Litz tested, there is no down side to a tighter twist than necessary, and there are even advantages.

As a barrel retailer, I think 1:7 is, all things considered, right for the 6ARC.