If these are for Phoebe 1810 are you going to be using Blomefield pattern guns which were standard issue from 1791 to the end of the muzzle loading era in the RN?9 Pdr Chase Gun Dimensions @ 7 ft 6 inches and 24 1/2 cwt (1:48 scaled c 48 mm)
I doubt very many care. but remember the designated length is from the muzzle to the breech ring, so the overall lengths are longer. The overall length in this case should be a little over 53mm.
