Elevation differences

[Iron Worker]

I have a 6mm Creedmoor with 7.5 twist . It shoots the Sierra110 MK under 1/2moa and the 108 Berger also under 1/2 MOA at 5,000 ft elevation . This June I'll be out side of Billings MT for Prairie Dogs at about 3,000 Elevation . The box on Sierra bullets says 7"twist needed,Berger says 8" twist . Is it possible the Sierra shoot great at 5,000 FT but not so great at 3,000 FT . Wonder if I should shoot the Berger 108s in instead ?

 

[Mikecr]

Twist requirements are declared at a standard atmosphere of 59degF and sea level, and some reasonable MV (like 2800fps).
3,000' already raises your stability from sea level more than you'll lose from 5,000'.
108BT, 8tw, StdICAO = Sg~1.45
108BT, 8tw, 3000' pressure altitude = Sg~1.59
108BT, 8tw, 5000' pressure altitude = Sg~1.69

So going from 5k to 3k causes loss of stability by 0.1
But, going from 0 to 3k raised stability 0.14 over Berger's recommended.

Good to go