I havent read all 5 pages, but AA 2520 is a pretty good powder for the 77's.
Accurates site has them going 2900 + FPS out of a 24" bbl.
bullet start FPS max FPS psi OAL
"2520 77 SIE HPBT 23.0 2,674 25.5 2,925 62,200 2.260"
This a "nato pressure" load. It dont say but I'd assume a 5.56 chamber.
From their site.
http://www.accuratepowder.com/wp-con...d_data_3.5.pdf
I'm not sure what you'd lose with a tube 4-6" shorter, but it cant be much.