Also remember that this was pre mass production. All firearms were hand made. There is no such thing as two identical guns from that era. Although the gunsmith may have been working from a pattern, the fact that each piece was hand made meant there would have been variations.
It was for this reason that each gun came with its own bullet mold - sized for that particular bore. Using a bullet from your buddy's mold may or may not have worked even though both guns were supposed to be the same caliber.