doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Lengthing chambers and safety limits

To me one of the worst things that a person can do is bore out the chamber of a in proof double gun. A friend asked me about this horror story of a 2 1/2 in. 20 ga gun. The chambers had been bored to 3 in on one side and 3 1/2 in on the other. I figured for sure the barrels were ruined. Not having a accurate barrel wall thickness gage. I did some calculations. At 3 inches it measured 0.700". I assumed a 0.150 thickness at that point and estimated that the barrel wall had been cut into 0.0425" leaving 0.1175". I believe the minimum allowable wall thickness at that point is 0.090" putting the gun 0.0275" within limits, but of course way out of proof..

I told my friend he needed some professional gun help as I was very uncomfortable with my calculations.

First question; are my assumptions accurate. Second question is there a fix here short of resleeving. I know Briley does chamber sleeving but don't know if that process would work here.

Pooch,

The first step is to measure the damages. If all is okay there, for $1000 you can send it to England and have it reproofed at 3-1/2" assuming there is such a chamber length in 20gauge. If it is accepted and passes proof, no major harm has been done.

However, if it fails proof or is rejected due to the modifications, a valuable gun has become worthless in much of the world and with discerning collectors.

This will be a learning experience for me, because all though I have a weakness toward junkers and will attempt most anything. I would avoid a gun like this.

The wall thickness will vary depending on the steel used, so there is no standard thickness.

The gun needs to measured properly. At this point it is out of proof under British law. Whether it is safe to fire is anyone's guess.



Pete

Pooch. Until measured, you don't know if any harm has really been done. It is completely acceptable to lengthen the chambers in the British system, is is unacceptable to have the gun returned without being subjected to reproofing at London or Birmingham.

There is a Brit (smallbore) named Diggory Hadoke that is in Texas sometimes, he could facilitate this gun being sent back to be proven in the UK and he'd tell you quickly if irrevocable damage has been done to the chambers.

There are many very valuable vintage guns out there that are proven for 3" modern magnum shells. So long as they are reproofed, it is a feature, not a detriment.

PeteM,

Is there such a thing as a 3.5" 20 gauge chamber length under British Proof laws? Meaning, if the barrels can be proven safe, can the proof house also make them "legal again" under British proof standards?

I had assumed yes, but I don't know 20ga gun chamber lengths.

Good question as the .090 is based on certain metals (I assumed metalugy available in 1920s)and the pressures at the point of cartridge discharge. With no such thing as a 3 1/2 inch 20ga, where would the point of discharge be calculated?

I will double check but I'm pretty sure there is no 3 1/2" 20-bore Proof.

The best you will do is 3", thought why the heck anyone does this kind of amateur 'gunsmithing' on good kit is beyond me.

You may be able to have the barrels chamber sleeved to 2 3/4" and proofed accordingly. Depends how badly the half a**ed job was done and what degree of concentricity the dullard has stumbled upon.

This sounds like I'm blaspheming but to me the rule of 96 is kind of like the Bible in that it was learned though years of experience. A man should pay attention and leave the chambers of a gun alone, they were built the way they were for a purpose.

Pooch, the Rule of 96 is a good guideline, but it didn't come down from Sinai written on stone tablets. Example: Per the Rule of 96, you shouldn't shoot a 1 1/4 oz load in a gun weighing less than 7 1/2 lbs. I've shot 1 1/4 oz hunting loads in guns at least a pound lighter than that, and while I would not want to use such loads for volume shooting (targets, doves, driven shooting), they're OK if you're hunting pheasants, the limit is only 3 a day--and, of course, assuming that the pressure is appropriate to the gun in question, which is the key issue. (From a comfort standpoint, recoil is also important.)

Greener came up with the Rule of 96 back in the days when, at least in Great Britain, all loads containing the same shot charge were more or less equal. That's why British guns, for many years, had a specific shot charge stamped on the barrel flats as one of the proofmarks. Well, my vintage Lancaster is marked 1 1/8, but were the chambers on that gun to be lengthened to 2 3/4" and were it to pass reproof, I still would not want to shoot most American factory 1 1/8 oz loads in that gun. Most of them exceed the pressures to which vintage British 1 1/8 oz shells were loaded, and they may well exceed even current British pressure standards, SAAMI service pressure being somewhat higher than standard British (CIP) service pressure.