Originally Posted by 1shot
I imagine there may be a small drop in efficiency and maybe consistency. Has anyone tried this?
This is correct... you don't need the oring, but it acts to seal the breech and ensure all of the pressure is on the ball and not escaping around the bolt. You will loose efficiency and will likely see more velocity fluctuation from shot to shot. I'll second what worrpt says, just use the smallest oring that will seal the breech w/o pulling out the insert - usually a 110 to 112 (you might have to stretch a 110 or 111), or a similar ID 1.5mm CS oring.