doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Proof house marking vs: bore size question

So here is a potentially silly question, but I will never know if I don't ask. When a gun that was made in the 1970's was submitted for proof, and was marked with .729 by the proof house, was that the EXACT measurement of the bores at that time? Or by marking it with .729, does that mean it falls within the 12 gauge rules that it must be in the .729-.739 range to maintain it proof.

The reason I am asking, is that I am looking a 12 guage that is marked .729, but has the following measurements: .732 right, .733 left and walls .028 right, .030 left.

While it is still within proof with those measurements, the wrench in this scenario is that this gun was proofed at 4 tons, and is a heavy waterfowler. It would be used with loads in the 10,000-11,000 psi range.

If this was a 6 1/2 lb, 1 1/8 oz proofed game gun there wouldn't be an issue, but my first instinct (which is usually the correct one) would be to pass on this one due to the wall thickness measurements. I guess I would expect a gun like this to measure .032+. Am I over thinking this, or would anyone else have issues with this one?

Go/No gauges are used at proof, bores are not measured.
In your guns case a .729" plug would go to a depth of 9", a .739"plug would not. Thus .733" could well be original.

That is something I did not know, thank you.

Any hesitations on a gun that is proofed at 4 tons, with .028 and .030 min walls?

Where, exactly, is that .028", and what is the MWT at the end of the chambers, the forcing cones, at 6", 9" & 18" from the breech? If the seller declines to share, I'd pass.
More information here
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

I thought by the 1970's they were somewhat more precise with the measurements than full 10 thousandths increments? Aren't they exact measurements now or are they still using plugs?

I had always been under the impression that the change from a gauge mark, as 12, 12/1 etc was to get away from this "Step" method & that when the decimal use started they were actual bore size, not somewhere twixt & tween so to speak.

That�s also my understanding.
The gun I recently had re-proofed follows the decimal marking to the tenth millimeter. The Rules of Proof 2006 also state bore shall be measured at 23cm using a tool capable of detecting 1/10th mm variation. So if it's a go-no go or a micrometer they're measuring the "start " point to a limited range.
Jeremy

Does anyone have a gun (or recall having seen a gun) from the "tons" era (1954-89) with an original proofmark that's anything other than the standard bore diameter? I could be misremembering, but as best I can recall, every British 12 I've seen from that era has been marked .729. Looking back at my records, I've owned W&S 700's from that era that were underbored by a few thou (.724 in one case, .726 in another), and I'm pretty sure both were marked .729. And with a slight underbore, it's obviously not the result of honing. I think .729 basically replaced 12 as the bore diameter during that period, but was no more "exact" than the old 12 or 12/1 etc marks.

Bartlett, is your recently reproofed gun (assuming 12ga) marked anything other than 18.5 mm, which is the current standard marking?

Not a 12ga Larry, but to affirm your post above see below a picture of the proofs on my 16ga W&S 700 in 16 ga...Geo

George, does that gun actually measure .662? Thanks.