doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Proof Mark Deciphering

I'm checking in to see if I'm getting this right.



I'm getting 1904 - 1925, 3" chambers.

The 13/1 is throwing off a little me as it seems to pre-date the BP/BV/NP under the crown marks.

Everyone uses that - which is why my question about the 13/1 came up.

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

.719-.728 is actually UNDERSIZE of standard 12 bore. Since you know what the gun was proofed at, by measuring it, today, you can decipher what amount of tampering with the bore has gone on over the years. That information is far more relevant than what year it was built.

The gun is a 13 gauge. There was a time when they were common, but, the wads were typically horsehair at that time.

Best,
Ted

I often do use Grok, and Perplexity to get started...

Your copied Grok reply illustrates some bell ringers for mistakes:

• There is no "FN inside a diamond"
• There are no "1/8 or 1 bits"

...and Ted's point about 13/1 being undersized being great examples.

Completely agree.