doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com What is considered safe min wall thickness?

Thin compared to gun made in the US? Yes. US made doubles are much heavier, thicker in the barrels if you want to call them that.

You do understand that pressures tend to be higher in smaller gauges do you not? Anything under .025 is thin in my books. If you are going to shoot only CIP loads or RST loads intended to keep pressure down you may be safe but with a small margin. If you are going to shoot US made, standard off the shelf factory loads, with pressure much higher than CIP then no it is not safe to shoot. The world is full of thin barreled gun which are borderline safe or not safe. What is your level of risk tolerance and what are you risking if wrong? Money or flesh?

Many (most?) English guns have been designed for light weight and fast handling. The barrel walls are made thin where pressure allows to reduce weight and improve balance and handling. With suitable English loads (which tend to be lighter and lower pressure and shorter cases than many American loads) they are as safe as any gun. Wall thickness requirement to survive the pressure varies down the barrel - peak being near the front of the chambers, and being much lower at the muzzles. Generally - barrels can be very thin at the muzzles from a pressure point of view, but would dent very easily.
20 thou towards the muzzles would be considered adequate, but will dent easily. However, much more is needed nearer the chambers. It is impossible to put an exact figure on this and expert advice should be sought if there is any doubt - and any advice given, particularly in respect of cartridge type and performance should be followed carefully.

Where an English gun is 'in proof' (i.e. not excessively enlarged in bore, pitted, altered or damaged since the previous proof) it will normally be safe FOR THOSE LOADS AND CASES FOR WHICH IT WAS PROVED. For many English guns that will be 2 1/2" cases and light (by American standards) loads.

British guns are all subjected to proof to see if they can withstand two charges in each barrel that are 25% higher than the maximum service pressure of CIP commercially produced ammo (which typically has a 15 - 20% margin of safety built in.

The thinnest safe barrel depends on what it is made from. Good steel will hold up better than poor quality steel of the same thickness etc.

If the gun is in proof and you use the right ammo, it will be fine.

I have seen guns made by the best British makers as lightweight 12-bores with original barrel minimums of 18 thou.

We in the Trade usually consider 20 thou a minimum for commercial purposes, which is why you see so many thin walled gun in auctions.

I have seen guns with damascus barres pass proof with a minimum of 7 thou.

I have also seen guns with thicker barrels fail proof.

Uniformly thin barrels will hold up better than thick barrels with steep gradients to thin spots.

There is no minimum wall thickness for proof testing. If the barrel withstands the test without material change, it will pass.

Dig, what are the reasons that a gun having a MWT of .007" would even be submitted for proof? Just to prove it could pass?

SRH

Guns should of course be used with shells which meet the criteria for which they were designed. This predominately though has to do with factors other than the minimum thickness of the barrel wall at some point down the barrel.
"Low" pressure shells is a term we have applied to shells having a lower Max "Chamber" pressure. This does not automatically equate to a lower pressure down the rest of the barrel. If two different loads push the same amount of shot out the barrel at the same velocity with one having say 10 K PSI & the other 8K PSI then those 2K PSI's have to be made up somewhere. They would thus be expected to be spread out over the forward portion of the bore thus would show a very slight "Increase" in pressure once about 4" from the breech has been passed. This is not to be construed as saying these low pressure loads are unsafe in a barrel having this wall down the barrel, but it also does not make them particularly safer on that account.
Total pressure can essentially be lowered by only one method & that is to lower the overall ballistics of the load, IE lighter shot charge, lower velocity or both. Max pressure can be manipulated by powder burn rate & a few other minor factors but as stated what you gain in one area you lose in another.

Miller, I think you either misstated something or else I'm not understanding. I've never seen a pressure curve that shows pressure increasing at any point down the bore. What does happen is that with some loads, the pressure drops off faster than with others. I can accept that your theoretical 8K load might have decreased less at 4" from the breech, relative to its initial pressure--that is, a pressure curve that does not decline as sharply--compared to the 10K load. But I'd have to see the results of a test in which the pressure increases, say, from the end of the chamber to the 3" point, then begins to decrease again.

Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, 1897
http://books.google.com/books?id=inQCAAAAYAAJ&source=gbs_navlinks_s

�Powder Pressures At Different Parts of the Gunbarrel, 1895� p. 32
http://books.google.com/books?id=inQCAAA...ure&f=false
(scroll down and click on p. 31)

The choice of 32 grains of Ballistite is confusing, as 24 grains is the published load for a 3 Dram equivalent; 26 grains for 3 1/4 Dram.
Alfred Nobel's 'Ballistite' was introduced in 1887, patented in 1888, then �Sporting Ballistite� (for shotgun shells) was patented in 1889, but not released to the trade by Nobel�s Explosive Co. until 1895. The published summary of the Trial states �Ballistite� rather than �Sporting Ballistite�.



1 1/8 oz. #6 shot at 1220 fps
Using the estimated Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch) (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI
Note: Modern transducer measurements average 500-1000 higher than LUP

32 Gr Ballistite Dense (1 1/8 oz. at 1232 fps = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624psi)
2 1/2" - 2.2 Tons = 6272 psi
6� - 1.2 Tons = 3718 psi
12� - .81 Tons = 1602 psi
18� - .33 Tons
24� - .21 Tons

42 Gr Schultze Bulk 1 1/8 oz. at 1220
Breech - 1.69 Tons = 4558 psi
2 1/2� - 1.49 Tons = 3886 psi
6� - 1.36 Tons = 3450 psi
12� - .92 Tons = 1971 psi
18� - .55 Tons = 1178 psi
24� - .32 Tons

83 Gr No. 4 Black Powder at 1222 fps
Breech - 2.26 Tons = 6474 psi
2 1/2" - 1.96 Tons = 5466 psi
6� - 1.26 Tons = 3114 psi
12� - .9 Tons = 1904 psi
18� - .37 Tons
24� - .27 Tons

An early morning (at least here in the desert ) rant

There is a very disturbing trend among dealers and auction sites (look through Julia's listings for this month) to post meaningless MWT numbers. WHERE is that MWT and how about the chamber, forcing cones, and the critical section from breech to 9"??
Measuring WT properly is hard, and takes the equipment, time (which is money), experience, and INTEREST to be done right. I've found some unexpected and frightening numbers, and those of us with an interest in vintage guns really have no excuse today for not measuring, or finding someone to measure for us, wall thickness on guns we intend to use.

What is your recommendation for this? The Hosford device that Kirby sells?

Gartenguy: an unbelievably talented and generous friend that is part of this Forum made me a Manson-style gauge. Works great and quite consistent. My only improvement was to affix a tape measure and make the pin slightly more 'pointy' in order to get in the bottom of pits.
It is very helpful to have an assistant help rotate the barrels which I affix to a weighted box (in which I carry the gauge and bore gauge) using bungie cords, and record numbers.







http://www.brownells.com/gunsmith-tools-...-prod20463.aspx