.577 - the 6 bore would of course shoot a ball, if patched in cloth, of about 6 1/2 bore, depending on depth of grooves. If a smooth bore, .030" or a bit more difference in bore/ball diameter in large bore guns, seems to work well. This allows a thick enough patch, to hold sufficient lube to keep the fouling soft. .035" (heavy) Denim works well in this situation.