As big as you can chamber, unfortunately without neck reaming, that will likely be .379" or .380" depending on the brass you use, .381" is better, but that won't happen without neck reaming. Soft .380" cast bullets will bump up, I've shot them in one of mine, but the other one won't shoot hard cast .380" which are tight to chamber in Starline long brass, keyholes IF they hit the target at 100yds, .379" jacketed shoot extremely well tho.
Tim