.5gr below the load on the 1lb container, starline brass, Fed210m. I too, only load 3 at a time as groups opened. My Henry was 1gr below the max and it shoots about the same size, 1in at 100yd
It has everything to do with with brass size. Every chamber and every gun is different. If you neglect to do your due diligence and make things right with lead bullets, you will get poor performance. I have a few rifles that have a bore of .450 and a groove of .458 but, I shoot .460 bullets...
Starline 45-90 brass was .001 thicker at the mouth and when trimmed to 45-70. You'll be fine with jacketed bullets. If it were me....its not and you choose to shoot cast bullets, buy a mold that is
.001 smaller than a fired case. It will perform much better with less leading.
will the new cartridge be based off an existing case? or is it new all together? RCC used to make just about anything but I think they closed. You could have a local machine shop turn some from bar stock.
for the 308, I have used 175gr SMK's, 210M's, hornady/fed brass, 44.5gr of varget, that load has very seldom let me down. In my AR10 I reduced it to 42gr same seat depth....030 off the lands