16ga.com ~ View topic - Minimum Barrel Wall Thickness -- Where?

We commonly hear about minimum tolerable wall thickness as one criteria for sufficiency of barrel strength, but I have never heard mention of exactly how close to the breech end that minimum can be located. Is there a barrel wall thickness "minimum profile" to which we should adhere? I'd like to hear from gunmakers and those familiar with the work of proof houses in regard to such minimum tolerable/advisable barrel profiles for various gauges, especially the 16. Thanks in advance for any insight you can offer, as I have not found anything definitive on this subject in the good old texts by such as Burrard, Gough Thomas Garwood, et al.

Joined: 19 Jun 2004 Posts: 1480 Location: Mpls, MN.

The proof house in St. Etienne, France measures the bore at 9" from the breech, and stamps that number on the barrel in mm. A deviation of 2 mm either side of that number is still considered "in proof".
If you have a 12 gauge, for example, that was stamped "18.3" and it now measures 19mm, 9 inches from the breech, you know some work has happened to the bore since it left the proof house.
Minimum wall is what will pass proof that day. French proof, is by law, the highest in Europe, and while they have discontinued the different levels of proof, it is about 1370 BAR for shotguns.
You don't tend to see .020 wall guns at the proof house in France.

Best,
Ted

Thanks, but let me re-state the question: 1) how close to the breech end of the barrel (the high pressure end) can the minimum wall thickness be. and 2) what is the minimum wall thickness profile from the breech end through the point of minimum wall thickness. Info on bore measurement and proof pressure is already available to us.

Ted, are you sure + or - 2mm is the bore tolerance? It seems so large. That's + to - .079 inches. That means a 16, with nominal bore of 16.8 mm (0.662") can be from 14.8mm (0.582") to 18.8mm (0.740"). That's about 24 gauge (0.579) to 12.5 gauge! Do you mean .2mm, or about .008"?

Joined: 19 Jun 2004 Posts: 1480 Location: Mpls, MN.

Yes, it is .2, NOT 2mm. Sorry about that.
I suspect that the measurement of 9 inches from the breech is telling you exactly where the minimum wall can be-all the proof houses in Europe measure at that same distance from the breech.
That said, I highly doubt there is a specific profile that every maker applies to his (or, her, in the case of the gun sisters) barrels, and said profile would vary with exactly what you were trying to produce that day-a 12 gauge light game gun, proofed for 2 1/2" standard European loads, would not profile the same as one with a 3" chamber specifically built to hunt ducks, that you were going to submit to the French proof house, and export to the US, for example.
When I was importing them, Darne guns with 3" chambers and magnum proof were built of a different steel than standard guns-regular guns and barrels were XT, a French tool steel, magnums were XTC, a steel not much different than 4140, saving a higher chrome content. It was a pain in the butt to rust blue.
I never measured them, but, I'd bet money the profile in the barrels was different, as the 3" guns were heavier.
I've also seen reports that the proof house in Birmingham has changed it's procedure for proof, requiring 2 tests in each barrel, in the case of a double shotgun. This change has caused a spike in the number of guns failing proof, and reproof.
The barrel profile that may have been the minimum before the new policy at Birmingham may just be under minimum for proof house testing today.

Things change.

With new non toxic shot regulations coming, cheaper shotshell components, like steel bases, instead of brass, and crummy factory powder that develops higher pressure sooner than the old powder, I have been leary of guns with altered bores and chambers for a few seasons. If there was ever a simple answer to your question of profile of a shotgun barrel (I kind of doubt it) I'm even more dubious that the answer would exist today.

Best,
Ted

Thanks Ted. Good. Makes sense. Material differences alone make a simple 9" wall thickness measurement a most superficial barrel strength criteria.