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

You understand it. This is a linear relationship so you estimates are good.

When you go below 0.020" wall thickness anywhere in the barrel you need to understand what you are doing. Likewise, you need to understand what fodder you are feeding it.

DDA

.030...better safe than sorry...

Ted if it�s .015 six inches from the muzzle I�ll risk it. Pressures should be low there anyways. Nothing that thin in the first 20�s for me.

I�ve gotten several enquirers about this gun. It is located in the Cincinnati area at a small dealer. So if you are looking at any 20 bore guns in that area check out the barrels first. Heck, you can borrow my setup if you don�t have one.

KYJon,
Now you have done it!
You have me worrying about shooting my circa 1900 Damascus barreled Parker 20 gauge.

Although, I have shot it since I was a teenager (sometimes with Remington Express and Winchester Super X ammo back then) and it still shoots fine. Could I consider firing that express ammo as proofing?

I only take it out bird hunting once yearly for old times sake as I usually hunt with my Parker Reproduction 20 or 12.
PS None of the Parkers are for sale.

I�m with Ted. There would have to be some compelling reasons to buy the gun if the walls are less than .025

I operate mostly on the idea that as good as this one is, there is always a better gun/deal to be had down the road. Might as well hold out for the ones that tick all the boxes.

It never ceases to amaze me all the folks here who will Brag on how lightweight "Their" guns are yet the have barrel walls seemingly made of Boiler Plate.
Sort of like listening to a bunch of Gamblers, the next day after a big night. one or two will have Won Scads of money, Everyone else Broke Even.

Crs, you have proof tested that gun many times by using the shells you did in the past. Buyers need to stop and think about what we are buying. For the most part these old guns are 100-125 years old. They have been used and abused. They have been repaired, freshened up and butchered by half ass repairs and half ass repairmen.

This was a very nice looking double that I expected to not find anything major wrong with. It needs to be sleeved or shot with tubes in in. The thin area is 14� down the barrels. That�s too close to both fingers and face for me.

Some barrels are thinner from the factory than you might think. I had a British 2� double and the barrels were .018 and .020. It was almost new, so I don�t expect any honing had been done to it. I have a DS 20 Lefever has .020 barrels. Same from the factory state, but I�m afraid it has too much pitting in it to be cleaned up and safely shot. Point is that light weight guns get that way by being made that way. The laws of physics and gravity still happen. So when I look at a six pound or less gun I worry about how thick the barrels are. Learn about it before you buy it because many sellers don�t want to take a gun back just because you found out later the barrels are a little thin.

It is my opinion that thin barrels past the proximal 1/3 are more likely to split length wise, rather than send chunks of shrapnel flying.
A chunk of just past the chamber burst.



When we choose to use vintage and/or second hand guns, without due diligence, we are however responsible for injuries to others...maybe our grandchild standing nearby?

Smith honed to .739" with resultant wall thickness of .018"; I don't know the load used

On a gun with 28� barrels the proximal third ends at about 9� which is where several of my fingers and thumb are. I often have a leather barrel grip and it is from the end of the fore end to another six to eight inches. Call it the middle third. In your second photo that would have cost me part of my thumb or all of it. If onvthe other barrel multiple finger could be at risk.

I do agree that burst in these areas do seem to be much more rear to front in nature like a seem opening up. But I want the thin areas if there must be any to only be in the last third of the barrels. Nearest the muzzle and furthest from me.