Hello all, I recently aquired a 410/12mm single shot H&R . I picked it up mostly for conversation and garden gun, but have never seen a 12mm on one. The chamber appears to be 2.5 in and from what I can tell it was produced in the early 1900s, probably before 1950 from numbers. Anyone have any input ?

Can't say for sure about 12mm marking but it does sound familiar. I do have in my collection a slip top box (circa 1920's) Winchester 32-gauge ammo marked " 14mm - 32 gauge" and the shells are marked 14 gauge.

its 2 1/2" 410 chamber