As said, the 55gn would be better in a 1:10 or 1:12 twist... 1:8 is better for 69gns

(My shooting buddy bought a Remingtom .223 with 1:12 twist, and the gunshop sold him some 69gn ammo... they went through the target sideways at 25 yds!!)

Any medium speed RIFLE powder will do... Something like Ramshot TAC is what I use for 55gns, and Vhit N140 for 69gns... but check published loading data...

Daryll