I would call this a gun bored for parer cases with a short cone. In 1875 a bore stamped 13 would take a .710" gage, but not a .729" one. A bbl bored specifically for an all brass hull would have been much more likely to have been an 11 ga which would accept a .751" plug but not a .775" one. Intermediate sizes as 13/1 & 12/1 did not start to be stamped untill 1887 in the British proof houses I believe. The thinner walls of the brass hulls required larger wads so bbls bored for them were larger. Prior to the advent of the plastic obturating wad a cone length of about 3/8-7/16" was fairly standard, to allow the wad to bridge the gap from case mouth to bore without gas escape. Many early cones as in this 1875 example were even shorter.
