It depends on the barrel length and bullets you prefer.
It is all about rotational velocity. 1:9 in a 20 inch barrel is not as bad as in a 7.5 inch barrel.
I would want 1:6 for a barrel under 9 inches, 1:7 for a barrel 9-20 inches, and 1:8 for a barrel over 20 inches.
Even for a long barrel length, I would consider 69 grain to be the max for a 1:9. If you want the capability to shoot 75 and 77 at sea level or in cold weather, then you need 1:7.
Is 1:7 significantly worse for 55 grain bullets? I doubt it. I have not seen anyone do a proper comparison test. You could order 10 barrels of 1:7 and 10 in 1:9 have have them made at the same time on the same cut-rifle machine and then compare them with 30 shot groups. That is under 1000 rounds of ammo.




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