A 12 bore paper case chamber size for 12 bore brass cases might well have .750" bores for sure - that would make it an 11 bore - heavy and perhaps for ball or shot?

I do not know what sized ball a 12 bore brass case that is fireformed, will hold.

M 16 bore brass easily held slightly undersized 13 bore balls at .705". Might have even held .710"s, true 13 bore size but didn't try it as the groove diameter was .705". The chambers were for 16 bore paper.(plastic fit as well)