doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Wall thickness 9" from breech?

What's the magic number for this measurement on a 12ga?

I know that down at the muzzle end we'd like to see .025 but isn't the number higher for the area 9" in front of the breech?


Thanks, Rob

I remember seeing a post that someone had a 12 that had passed English proof with about .020 wall thickness in that area. Now then, it escapes me as to what type of proof it was. One can still request black powder proof there.

I have a short tubed (25 1/2 inch) 12 gauge Darne that is packing .080 in that area, and, even with a pimple bulge or three, I shoot it with anything I've got that isn't marked "Magnum" or longer than 3". That, seems to be an extreme wall measurement, outside the chamber area, on double guns from what I have seen.

I'm not sure what a rule of thumb measure for minimum would be, or how relevent such a measurement would be from gun to gun, but, I would expect at less than .025-.030, alarm bells should be going off.

Best,
Ted

From the Griffin & Howe site:
http://www.griffinhowe.com/abbreviations.cfm
"A rule of thumb states that the minimum barrel wall thickness should be .020" in a 12 gauge gun." per the British Gun Trade Association

From the PGCA site:
http://www.parkergun.org/forums/forum1/595.html
We who shoot Damascus barreled guns look for a minimum of .025 wall thickness...and .080 at the end of the chambers.
Customary measurement is 9" from muzzle and breech.

UK "Working Standards for Re-proof" minimum wall thickness measured 18" from the barrel breech from Double Gun Classics p. 56, Vol. 1, No. 4, Jan-Feb, 2006':

2 1/2" 12g- .028
2 3/4" 12g- .032
Re-proof recomended minimum- .024

Thank you men.

It is very dangerous to be pedantic about wall thickness measurements: I have successfully nitro re-proofed both steel and damascus tubes down to 0.014" MWT. However, I wouldn't recommend this as a hobby!
The thinnest part of a barrel tube is likely to be at about half way but again, it depends on what the original, or subsequent, barrel filer was trying to achieve in regards to finished weight and balance. Often this thin patch will be only partially round the tube: the area beside top and bottom ribs often show a marked thinning.
Beyond this half way point, the wall thickness will usually gradually increase towards the muzzle by up to 0.010-0.020". This is partly brought about by the choke constriction but even with cylinder chokes, I would expect to see a 0.010" increase in all but muzzleloader conversions.
I have always been lead to believe that this is to help protect the susceptible muzzle area from dent damage and also increase the swing momentum in lightly barreled guns.
Thickness at the other end of the tube is obviously of much more important in regards to safety and lightly barrelled guns with lengthened chambers are often considered a real hazard.
I have never seen nor heard of a barrel bursting due to this treatment but I will admit that it must increase the risk of a blockage burst.
As to the minimum thicknesses of the tube wall at 9", I personally wouldn't worry about it in a gun that has (near) original and sensibly tapered tubes from breech to muzzle and has not been lapped out beyond its nominal gauge range.
Put it another way, if the minimum wall thickness elsewhere is at 0.020" or over, the tube is internally parallel until the choke constriction and the barrel hasn't been seriously messed about with on the outside: Relax.

I believe the measurement taken by the proof house 9" from the breech is for bore diameter, not wall thickness. However, I would certainly expect the walls near the breech to be significantly thicker than near the muzzle.

Thr reason I ask is the gun in question, a 12 has bores of .745 and .740, definately out of pro0f for a 12 in diamond marking. However the walls are .082 & .085 9" from breech, .038 & .037 at the mid point of the barrels and .027 &.032 9" from the muzzles.

The gun would be in proof if marked 12 over 1 in a diamond, overbored.

Am I correct to assume there's enough here to reproof and will the proofhouse re-proof an overbored gun?

Joe: the Belgian proof house measured the bore diameter 22 cm from the breech 1898-1924 per Bruno Joos de ter Beerst's Trade & Hallmarks on Firearms in Belgium.

My feeling regarding wall thickness is that the critical area is the end of the chamber. Here are my standards:

gauge chamber end diameter minimum wall thickness
12 .800 .085
16 .732 .075
20 .610 .070

The measurement at 9 inches from the breech is for a bore gauge. There are ranges of tolerance of departures from these diameters, but I use the following as benchmarks for 12/16/20: .729/.662/.615

I don't buy a gun that has chamber wall thickness below these minimums, and I prefer bores that only exceed the bore dimensions by .005 to .010, depending on the gauge.

It appears that there are already plenty of opinions on barrel wall thickness ahead of the breech, so I won't add my two cents to that.

This is for what it's worth.

Note that the 22 CM measurement converts to 8.66" or very similar to the British 9" point. In older muzzle loader days bore was measured at the muzzle. With breech loaders & choke boring this point was determined as the point of measurement. Burrard mentioned that many British guns were "Enlarged" forward of this popint in regulating chokes for a given load at the factory, when New. In efect a Looooong jug choke.
If that were my gun unless I were planning on selling it in Emgland I would just shoot it with appropriate laods, not bother with the expense & trouble of a re-proof, though I see no reason based on the bore & wall they would not do so & mark for the new bore size.
Note also the 12 in a diamond simply denotes the nominal chamber of the gun & not the "Proofed" bore size. If British there should be a non-enclosed mark for bore size as 12 or 12/1 etc. If Belgian, German etc will have a mm mark for bore as 18.5 etc.