The 12.7 or .50 cal is virtually a 32 bore. Yeah - kinda small.
I think I sold my Husky M20, 16 smooth SXS 13 bore rifled (Cape gun)for $275.00 CDN. IIRC.

I refinished the stock's faded and almost missing varnish!
had some nice walnut afterall.

Of course, I slugged the rifled left bore, which had straight rifling. It measured .705", yet perfectly fitted 16 bore brass shells. The partially marked ball is a .682" - the bore was .670" and the groove was .705".

I will note that the denim patched 16 bore balls, were more accurate than the 14 bore balls shot naked. in the 13 bore barrel, which is understandable.

The .662" balls were patched with 12 ounce denim which measures .030" with Dial calipers, measuring .663" ball + /030 + .030" (both sides) = .722" = which gives some compression in the grooves. So - clean shooting and best accuracy.

Incidentally, this was shot offhand at almost 30 yards.