I've never used the 77gr SMK, but from pictures it looks quite longer than the Hornady 68gr BTHP.
And my 1/9" Bushy is not even close to stabilize that. Using a 150 constant, the longest bullet a 1/8" can stabilize is .9408, to go up to 1" using 165 (that I find to work fine with 223 velocities). The Hornady 68gr BTHP is .984, and it's a 68gr bullet.
I expect the 77gr SMK to be over 1".
Leaving the math apart, and to answer your question, I always shot 62gr bullets with my ARs (all 1/9") and I've no plans in going down with the grains, and I do not believe in over-stabilization and/or disintegrating bullets.
A 1/8" barrel might do it (will definitely work on the Hornady 68br BTHP) as well, but I'd be slightly happier with a 1/7".
Is it known for a fact that 1/8" is dead-accurate with the 77gr SMK?
- Ice