doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Thin barrels passing London proof

I agree with tut (on page 2), if I set minimum wall thickness at an arbitrary .030 near the muzzle there are many in my safe that should go to the junk pile instead of the field. I have a number of Parkers, for instance, that can't pass that test and are definitely untouched. This discussion will never be settled--it is strictly up to the individual and in a sense neither side is wrong. Where I do get real skittish is in the chamber area and the next couple inches. Even there though I often drop below the commonly accepted dimensions. My gun, my fingers.

Not trying to be a smartalec but your logic is a little bit off. All else being equal, wall thickness etc. the smaller diameter tube will withstand a greater pressure. Not an insignificant difference either.

Years ago a story was told of a man who had a Model 12 trap gun that was so worn there was a pinhole at the end of the barrel. When he shot, a little puff of smoke came up out of the hole. Everyone figured there couldn't be any choke left in that barrel, but he believed in the gun and he broke targets with it. Finally when the gun was in to be repaired for something else, the gun smith welded up the little hole. When the gun was given back to the man, he said he's kind of missed the little puff of smoke.

Thank you Blue Grouse & Toby Barclay for straightening out misinformation posted early on in this thread regarding MWT and Proof House.

I owned a gun a couple of years ago that had just passed London Proof with a MWT of .018.

Miller, I think I may be missing something here. Seems to me what you wrote can be interpreted as saying that pressures on low pressure loads INCREASE after they leave the chamber. I've never seen a pressure curve that goes in any direction other than down after leaving the chamber. I think you're trying to say that pressures on higher pressure loads using fast burning powders drop off faster than they do on lower pressure loads, especially those that use slower burning powders. But whether the lower pressure load's pressure is higher than a higher pressure load at any specific point on the curve depends on the specific loads and how much difference there is between them to start with. I think the only generalization that can be made is that lower pressure loads often show a slower loss of pressure than do higher pressure loads.

1cdog - In one of our previous discussions it was reported that the Birmingham Proof House would notify the owner if the gun had a wall thickness less than .020". Was that done with your gun? Did you receive a copy of the Proof House certificate? Could you please post an image if so? Thanks.

Larry;
Quite possibly I didn't word my statement perfectly,though I thought every one could understand what I was saying. Yes it is totally true that pressures falls for the rest of the barrel's length once the max chamber pressure has peaked. "However" f the same ballistics are kept those pressure curves will cross in the area from about the end of the chamber to around 4" from the breech.

From that point forward the "Low"pressure load will have a higher pressure than will the "High" pressure load. This was intended as a Reminder, note the word "Remember".

Pictures of these pressure curves have been posted Here on This forum many many times & they have been thoroughly discussed. I truly felt like anyone but an "Idiot" would understand what I was saying, You "DID" understand didn't you.
Yea!! I though so you just proved beyond all doubt that you are qualified.

Us non-deep thinkin' types do better with pictures

Sherman Bell�s pressure testing published in The Double Gun Journal Summer 2002 �Finding Out for Myself, Part VI, Smokeless vs Black�, p.19, and summarized in Volume 17: Issue 4, Winter 2006, p. 39
1 1/4 oz. 3 3/4 dram (102 gr.) GOEX FFFg Black Powder at 1240 fps and the equivalent load of Blue Dot. That is a boomer load few of us are likely to be using in a vintage double

................1 inch........6 inches.......12 inches
GOEX.........5900 psi......4100...........2100
Blue Dot.....6000..........4300...........2300

At 6" modern "non-progressive burning" powders are about 1/2 of Blue Dot's and GOEX pressures, and at 12" well below 2000 psi. Alliant data for 1 1/8 @ 1200fps

Alliant e3 - 17.2 grains = 10,950 peak psi
Red Dot - 18.3 gr = 10,300 psi
Green Dot - 21 gr = 8,900 psi
(Infallible) Unique - 22 grains = 8900 psi

we talk a lot about minimum barrel wall thickness, but what we really should be talking about is barrel wall thickness in front of the chambers, where the maximum pressure typically occurs...my gunsmith, old ed, recommends not firing a 2 3/4" chambered 12 gauge gun with less than .090 thousands in front of the chambers...what say you?

Ed, Am I reading the charts wrong or are you? They all seem to show the maximum pressure well within the chamber, not in front of it.