doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Shotgun Barrel Wall Thickness

I started this discussion regarding a customer's H&H, but I'd like to get broader input on the subject.

For years I have been using a guide from an old gunsmith on barrel wall thickness at the end of the chamber as follows (all in inches of course):

Gauge Chamber End Dia. Min Wall Thickness

12 0.800 0.085
16 0.732 0.075
20 0.610 0.070

Is this sufficiently conservative for chamber end wall thickness?

Since the British measure bore diameter 9 inches from the breech, is there a consensus on minimum wall thickness for that point?

Would most people agree to a minimum thickness 9 inches from the muzzle at 0.023 to 0.024?

I realize that bore diameter affects the hoop strength of the cylinder, but I am primarily talking about standard bore diameters of 0.729, 0.662 and 0.615 inches, respectively, for 12, 16 and 20 gauge guns. What is the consensus on overbored guns, for example, 12 gauges approaching 0.740 bore diameter?

Allan

Allan, I guess this statement; Would most people agree to a minimum thickness 9 inches from the muzzle at 0.023 to 0.024?, would depend on what type of hull pressure shell you are using. I
would say that a low pressure, low recoil shell would be fine. I know that some of the British guns have a thinner wall thickness due to keeping the guns light.

In guns that have been honed out and in your example of going from .729 to .740, that .011 would be .0055 less wall diameter meaning now .024-.0055 or .0185 wall thickness, and yes I would say that is too thin and would definitely be out of proof.

I want a bit more wall thickness in my damascus guns, especially in the area immediately in front of the forcing cones. Can't say about fluid steel--it hasn't been invented yet in my mind.....

Without looking anything up, I would want MORE thickness for the smaller gauges simply because the pressures are higher, in them. I like to see about .160" @ breech, .095" or more at the beginning of the farcing cone(end of chamber) in a 12 gauge. I would not like less in 16, 20, 28 or .410. Again, because the pressures are higher, as the gauge goes smaller. JMO, of course

I would say your initial wall thicknesses are too Low!!
G T Garwood quotes an average of .114" for end of chamber on English 12ga Guns from the mid 20th century. He goes on to point out some recent ( c@1970) European Guns that Average .084" but uses the yield point of 32 imp tons of superior barrel steel to demonstrate that this low thickness is at the bottom end of the safe range.( See shooting facts and fancies Ch34 for detail).
At 9" from Breech I like to see .040" and in the run ( 10-4") back from muzzles .025" will give sufficient thickness to reduce risk of denting. As noted above an increase in thickness is warranted in sub 12 gauge guns.

FWIW, I have a circa 1890 Birmingham hammer gun with "London Laminated Steel" barrels in 12 ga that appears to be original. Walls ahead of the forcing cones are .130", which I thought was a bit much until I read this thread. Seems OK now.

Just to be certain we are all on the same page in saying "Ahead Of" the forcing cone are we speaking of the juncture of the chamber with the cone or the junction of the bore with the cone. The point where the chamber meets the cone is generally the most critical point in a shotgun bbl. Unless a very long cone the cone will normally taper more than the OD of the bbl so "Ahead of the cone (as in front of it/forward) will be thicker than behind it (breech side) as well as the pressure will be lower there than at chamber end.
A book I purchased many years ago (copyright 1966) entitled "The New Hunter's Encyclopedia" had some figures they quoted as being from the German proof houses which stated min wall at forward end of chamber should be in 12, 16 & 20 ga's 2.3mm ((.0905") & for the 24 & 28 ga's with their higher pressures 2.4mm (.0945"). This was stated to be with "Ordinary Good Steel" while if "Sppecial Steel" was used these could be reduced by no more than .2mm (.008") in the 12-20 ga's. No mention of reduction for the smaller bores. It was further stated the minimum thickness in the forward third of the bbls should be .6mm (.024").
Certainly bbls have passed proof with thinner walls in the forward portion, but personally I would not feel comfortable with any less in the chamber area & more would be better.

Given Miller's comment concerning potential confusion over the term "ahead of the forcing cone" (which direction is "ahead"?), it's probably better to use "end of the chamber" as Hugh Lomas did in his post.

Miller, I have the same book you cited. Forgot how much good information it contains! However, mine is the 1972 edition, and it looks like they may have edited out the information you posted on wall thickness, etc. Can't find it anywhere in the shotgun section. Where did you find that reference in your book?

Larry;
This may or may not be compatable with your edition but in mine it is in section X Firearms/ Rifles / Principal Foreign Proof Marks beginning on page 546.

Thanks, Miller. Found it. I skimmed over the proofmark section the first time and missed it.