...I also noticed that the groups tended to open up if I didn't "hold" the rifle. It seemed that if I let it recoil freely, the shots moved or opened up on some of the hotter loads.
I shot back to back groups with my Montana (338 Fed) using the same ammo, and free recoil resulted in ~3" groups, holding basically like shooting offhand, but with the bags to steady things, resulted in .75". Free recoil and Montanas do not go together. In my opinion... Now my .308 with a 26" heavy barrel is a different story.
