I have one and am still playing around with loads. The bore slugs at .3135, but I have heard people state that some are .316, which seems a bit extreme to me. I have been getting three rounds that touch, and the next two seem to open up the group to about 1.5". Fine for hunting. There was a Handloading Magazine last year that had an article on the 303, and used a Number 1 for their tests. They had some excellent results. You can get .311 and .312 jacketed bullets, and if you want to try cast, I believe you can get them sized up to .314 from some places. I'm satisfied with my results so far with the jacketed, but may try some cast this summer.