doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com lengthening chambers

I saw it as back-locked Jones under-lever...my comment about it being a screw grip was a joke.

The well informed responses to my post have convinced me that I've been wrong in my opinion about chamber lenghtening. Well mostly wrong anyway. What bothered me are the following facts. SAMI allowable service pressures for 2 3/4" shells are nearly 2000 psi more than CIP pressures for the same length shells. So a 2 1/2" gun is deepened and reproofed to CIP standards. Exported to the USA and then used to fire SAMI shells. Where is the sense in that? The gun has been weakened to allow the use of higher pressure shells. Wouldn't it have made more sense to just lengthen the forcing cones? No doubt the people who do this work have measured the barrel contours, etc. and have concluded there's metal to spare, well maybe just eyeballed it but what the hell, it's only a quarter inch. Don't get me wrong, when I was a kid I made 12-bore shotguns out of 3/4" iron pipe slid inside a 1" piece. Killed as well as a H&H Royal. It's just that in my dotage my opinions are less and less rational. I'm glad I can count on the folks here to help me keep a grip on reality.
npm

Larry, that's obvious, I didn't state anything to the contrary.

I have a 1939 Nimrod drilling that has 70mm chambers but is not stamped as such. I had the gun checked thoroughly, was told it had no damage. I only shoot 2-1/2" in it to keep it that way. I have no idea how much the gun was shot after the chambers were lengthened (my guess is not much at all) or what shells were used. Very few buyers can be sure what a gun in question was fed before they owned it. My point being that just because a gun has lengthened chambers , by and of itself, is not a reason to condemn it. What needs to be taken into account is the result of actually being shot and what was shot.

Now, how chamber length originality affects collector value and price is another story. If they're not original I use it every chance I get to lower the price on a gun I'm buying. If they are original it's definitely a point to bring up if I'm selling.

Rob, my first classic double was a pre-war Sauer 16. Original short chambers. Had them lengthened, hate to think about how many 1 1/8 oz pheasant loads I pushed through that old gun. Didn't hurt it a bit . . . but it's one of those case by case deals. I would not worry too much about lengthening chambers on most older American guns, as Jerry suggests, because they tend to have thicker barrel walls than Brit/European guns. Probably the best thing on a gun with punched chambers is to have wall thickness measurements taken at the end of the chamber. The acceptable minimum I've heard, at that point, is .100. I'd have a fair degree of confidence if there's that much steel left.

Nial, per the Birmingham Proof House, the difference between SAAMI service pressure (11,500 psi for a 12ga) is less than 800 psi higher than CIP service pressure (10,730 psi). The Brit "bars" figures are crusher bars, not transducer bars. Converting to transducer bars--again, per the Birmingham Proof House--CIP service pressure is 740 bars rather than 650. Essentially, what that means is that current standard CIP service pressure is very close to American service pressure on our own old, short chambered guns--prior to the advent of SAAMI and the conversion to standard 2 3/4" chambers.

npm
I'm glad I wasn't the only kid who made a 12 gauge shot gun out of a piece of 3/4" cast iron pipe. Thank you.
Steve

Thanks Larry. Accurate information is always appreciated. Just now for curiousity I measured the chamber walls on a 16 gauge 2 1/2" Brit gun. They look to me about average in that chamber walls are 5/32" at the breech then taper down to .096" right at the start of the cone. If I were to deepen them .250" the minimun chamber wall thickness would be pushing .085" I think this is a typical English 16-bore game gun, not extra light, weighs a little under 6 lbs. .085" would probably be OK in 16 gauge, smaller diameter tube= stronger, etc. But what if it was a 12 gauge? What then? You mention a tenth of an inch as being what you would be comfortable with. I guess my point was, I see lots of Brit game guns that have been deepened and reproofed, but why? It doesn't add anything to 'em. If the cones were just lengthened people could go ahead and fire the 2 3/4" cartridges in a stronger tube with little or no extra increase of pressure. That's my objection to lenghtening chambers right there in an nutshell. What's the point? Pay a bunch of money to have "Reamer Rube" do his thing, then another bunch to the proofhouse but end up with less gun.
npm

Nial, I agree with you on the lengthening of cones but not chambers. And as long as the 2 3/4" shell is loaded to a pressure appropriate for the gun in the first place, you shouldn't hurt it. However, I'd also point out that it's much easier to work up low pressure reloads for the 12 (5-6,000 psi range) than it is for the 16.

I've enjoyed this thread so far. I just have one thing to add. If you plan to shoot shells loaded with fiber wads, BP or smokeless, lengthened forcing cones may allow powder gas to leak around the wads, where lengthening the chambers with a chamber reamer that has a normal forcing cone taper, won't.

Here's a really good quote from Chuck H. which everyone considering grinding out and butchering chambers in the older guns should read and ponder prior to going under the tool for permanent change .......and why to never buy a gun that has been altered in this fashion.......

Even if the outside barrel contour doesn't have a taper, reaming is going to reduce wall thickness. So will honing to take out pits. You remove metal, you have less metal left. The question is, how much did you have to start with, and how much is left when you're done. Lengthening chambers only 1/4" (as on Brit/European guns with original 2 1/2" chambers) can be perfectly safe; witness all of those to which it's been done, and which have passed reproof. Most of the time, however, you'll get the same benefit, with far less metal removed, by simply lengthening the forcing cone.