doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Thin and OMG thin

Was looking at a 20 bore double and started measuring the walls for thickness. It a very light weight British double. All was fine until I hit a long patch of .011-.010-.013. By long I�m talking about six inches. And I know we often say most thin areas are OK if near the muzzle. But that thin, is just too thin for me. Shame because it had such beautiful Damascus barrels. Seller claimed not to know how thin they were and maybe he did not, but he does now. Even with �proper� loads I would not shoot it. I considered putting a set of 28 tubes in it but you go from a 5 pound 7 ounces to about 6 1/2 pounds and it would be very muzzle weighted.

A setup to measure barrel wall thickness is an absolute must have part of my equipment. I won�t buy a double anymore if I can�t measure the barrels. I�ve had a few sellers who did not want me to measure the wall thickness. Part are afraid it will damage the finish, some don�t understand why I care and a few I think don�t want to take a chance of finding a hidden flaw. So if the seller refuses to let me measure them I just thank him for his time and walk away.

Wise move indeed-- lotsa strange things could have happened to those barrels over the years--RWTF

20 gauge pressure runs a bit hotter than 12. I wouldn�t expect what passes for minimum in a 12 to be the same for a 20.
A damascus 20 is likely a pretty rare thing. Truly a shame.

Best,
Ted

Note that the formula for hoop stress (the force that would rupture the barrel at the weakest point exposed to the pressure) is pressure times diameter divided by two times the thickness of the wall. Directionally speaking, a 12 bore experiences higher hoop stress for equal pressure than a 20 bore and a
said 20 bore experiences lower hoop stress than a 12 bore for the same pressure. The 20 bore will require higher pressure history (the internal pressure curve) than a 12 bore to achieve equal velocity. My point is that rather than comparing 20 bore wall thickness to 12 bore it is better to work the math.

DDA

Rocketman, would you actually buy and shoot a set of 20 bore barrels that were .011"? If not a risk for bursting, they are certainty a risk for denting and even reviling I suspect. .010 is almost paper thin to my eye.

Probably. But, just me, I guess, a .010 MWT 20 gauge is beyond my desire to work any math upon what remains of it.

Best,
Ted

No, I wouldn't. Period. My point is that the math is easy enough that we really don't have to rely on comparisons between 12 bores and 20 bore guns.

0.010" wall 20 bore at 1000 psi and bore of 0.615 = 30,750 psi

0.018 wall 12 bore at 1000 psi and bore of 0.729 = 20,250 psi

0.010" wall 12 bore at 1000 psi and bore 0.729 = 36,450 psi

The 20 bore has 84% of the hoop stress for a 12 bore of equal wall thickness. But, the 20 bore has 162% of the hoop stress we might expect of a 12 bore at usually recommended minimum wall thickness.

DDA

With your experience, perfectly reasonable. I still think it worthwhile to understand the quantification.

DDA

So what does the math tell us? That a 20, with walls with a thickness of maybe .015 is about as strong as a 12 with walls of .018. Just looking for a basic handy rule of thumb number.

My rule of thumb, which is just a general rule, was that I am not interested in 12 bores with walls thinner than .018 to .020 or so. And that has to be in the last third of the barrels. Not near my favorite fingers. Not for fear of bursting as much as denting. In 20 bores I drop the thickness to about .015.-.016 as my lower limit. Had this gun been in the .015 plus range it would have gone home with me. The barrels were such a great looking set of Damascus barrels.

Jon,
I�m a lot more comfortable at .025 than .020, or .015. You were looking at a Damascus barrel gun, and would consider it at .015?
You are a braver man than I am. 12 or 20 gauge.
A modern, steel barreled gun might be fine at .020, but, I guess I�m not interested.

Good luck.

Best,
Ted