Yes - I agree - the 15 bore Manton Cap gun shoots like a properly fitting modern double. It swings so quickly, birds are easily taken on the early rise, when the pattern is still perfect - inside 25 yards. The bors are straight - no tightness in the middle as some Manton's apparently were bored. It was commonly thought that a choke, that is, a constriction which was relieved towards both breech and muzzle made the gun shoot harder.
I cannot see that myself, however I've not shot such a gun, so have no experience with one so bored.