Joe: Your answer is probably better than I could have given. The minimum thickness I wrote about comes from discussing the issue with several BP gunmakers, and was the result if reading an article in The Buckskin Report anout a replica Hawken style rifle that had the barrel rupture where a key slot was cut too deep in the bottom of the barrel. Compounding the problem, the grooves in that barrel were cut too deep, leaving less than .020" of metal between the bottom of the groove, and the slot! :shocked2:
I suspect that I have discussed this issue with just about every machinist, gumsmith, and gun maker I have met over the last 40 years, because I am always interested in their take on the subject, and whenever new steels are being used for barrels, its worthwhile considering if you can get away with a thinner barrel.
I have some VERY THIN muzzles on 19th century shotgun barrels , but as suggested, you will usually find more Metal around the barrel back towards the breech. Two Tenths of an inch is a LOT OF METAL, and I don't see that much metal in most breech loading shotgun barrels! :shocked2: My fowler was made leaving extra metal at the muzzle and rounded end of the barrel so that I could have the barrel jug-choked if I choose to do so. But, the rounded portion is slightly tapered from the wedding bands to the muzzle, leaving it much thicker back at the bands, and even thicker in the back, octagon portion of the barrel. Your highest pressures are going to be at the rear of the barrel, with pressures dropping as the powder burns, and the PRB or shot load moves forward towards the muzzle.
The problem with the gun that John Baird covered in The Buckskin Report was that it had two barrel keys, putting one of them only a few inches ahead of where the PRB is seated on a substantial load of black powder. The Weak spot above the key slot simply could not hold the pressure at that point, and the barrel ruptured. The stock was split, the ramrod broken, and the key blown out and bent. There was a long split in the bottom of the barrel, too, IIRC.
We are fortunate that today's barrels are made of modern steels, and that the steels chosen can pretty well anticipate "mistakes" by owner that would have blown up an iron barrel 200 years ago. Those mistakes can be contained by the new barrels. You might bulge these barrels, but its much less likely that the barrels will rupture and send metal and wood to harm or kill anyone.
The nightmare that barrelmakers live with is the guy who builds his own gun, without realizing that all those parts he buys have extra metal on them, so that he can file them to required dimensions, retaining a margin of safety. Unless you ask the question on this forum, as the poster of this thread has, or happen to read a forum like this one, ignorance can destroy weeks of work, and cause permanent injuries.
Its not the barrelmaker's fault that someone cuts their dovetail slots, or the grooves, too deep.
:nono: :surrender: :thumbsup: :thumbsup:
