You want 2900-fps with a 75-77 in a 20" .223? Remind me not to shoot next to you!
Seriously, you can drive a 77gr Nosler/Sierra to about 2750-fps reliably with a 20" 1-8" twist barrel. The 80gr bullets can be driven to about the same speed but require adjusting seating depth and bullet jump.
As for sacrificing speed, I have it from a reliable source the Army Marksmanship Unit uses an 80gr .223 load at 600-yards that runs slower than what most of us civilians shoot. They seem to do alright with it...
