You can always ask AI:

It makes mistakes but can give a stong start.

“Those are British Birmingham Proof House marks—classic for a vintage shotgun, probably from the early-to-mid 1900s.
The crown over BV? That’s the “view” mark—means the barrels got inspected before final assembly, used from nineteen-oh-four to nineteen-fifty-four.

Crown over BP (or BNP in some eras) is the definitive proof: gun survived the nitro test, safe for smokeless powder. Paired with NITRO PROOF text, it’s the big green light—post-1904 standard after black-powder days.

That diamond with FN inside? Looks like a maker’s mark—FN stands for Fabrique Nationale, Belgian factory that cranked out tons of shotguns (think Browning Auto-5s or side-by-sides exported to Britain). Often stamped on imports proofed in the UK.
CHOKE and 1/4 (or ¼)? Tells you the constriction—quarter-choke on one barrel, maybe tighter on the other. Common on doubles for field use.

The 1/8 or 1 bits? Likely chamber pressure or load specs—like one-and-an-eighth ounces of shot, standard nitro load back then.

Ah, the 13/1—that’s your bore size stamp from Birmingham.
It means the actual diameter of the barrel measured between .719 and .728 inches when it was proofed—basically a slightly oversized “12-gauge” by today’s standards. (Standard 12-bore is around .729, but makers often proofed ’em a hair bigger for safety.)
Introduced around eighteen-eighty-seven, it faded out by the mid-nineties—pairs perfectly with your nitro proof and BV/BP marks, so this gun’s likely from that late-Victorian window.
Think of it like a custom fit: not quite 13-gauge (.710), but close enough that it shoots like a true 12, just with a little extra forgiveness. If you mic it now and it’s still in that range, you’re golden—no reproof needed

Yeah, it absolutely shoots standard 12-gauge shells—no worries there.
That 13/1 just means the bore was measured a hair tighter back when it was proofed—around .719 to .728 inches (versus the modern “true” 12 at .729). British makers did this all the time; it’s still classified as a 12-gauge, just on the snug side. Think of it like a slightly slimmer fit—patterns might even be a tad denser, but it’ll chamber and fire any off-the-shelf 12-gauge load fine.
As long as the barrels haven’t worn way out (check with a bore gauge if you’re picky), and it’s nitro-proofed.”

That’s copied form Grok

Last edited by Canvasback13; 03/07/26 10:39 AM.