Welcome to the forum.
.56 is a new one on me. Never heard of one. A 28 gauge is .550 give or take a few thousands. Dixies catalog shows a 26 gauge as being .563 in their proof load listings but who ever heard of a 26 gauge?
The standard calibers above .45 are: .50 (shoots .490-.495 round balls), .54 (shoots .530-.535 round balls), .577 Enfield, .58 (shoots .570-.575 roundball or minie bullets), .69 and .75.
I would suggest you have the bore measured. It most likely is a .54 with a mis-stamped caliber marking.