I formed some 6mm/303's tonight. They hold 46.0 gr. of stick powder. I tried the same powder in a fired 243 Winchester case and it held 48.0 gr. (about a 5% difference, but the gap will close somewhat when the 6/303 is a fired case too)
I figure to load about 1 gr. less than the recommended starting load for the 243 in the 6/303 to start with. Sound right?
Lefty

Thanks for the info.