The gun is marked 14B over 15M.
So clearly this is a 14 gauge that has been over-bored. Which I guess wasn't that uncommon.
Thanks for the replies.
This is a bit of a red herring, you are quoting the 1875-87 proof marks commonly accompanied by 'NOT FOR BALL'. However this is the proof size (the 14B; the 15M is the muzzle size giving the choke constriction) NOT the chamber size. Sadly at this time, there was no chamber marks (in a diamond) so the only way to tell what the chambering is to measure it or use some kind of Go/NoGo gauge eg catridges of the various different gauges. Note that there were some very odd sizes promoted by some gunmakers like 14 1/2 bore by Lancaster I think. There may have been others. At this time there was very little standardisation and ANY chamber size could be encountered albeit rarely.
Enjoy!
