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.

THe rule of 96 is only valid for 12 or 16 ga. guns, if then. If you apply it to .410 bore guns with a 1/2 oz charge, you get 3 lbs, and other gauges give similar impractical weights.

Come to think of it the French got around the 96 rule with their Darne and with using swamped ribs. But the 96 rule is still a good one and I'll abide by it. The idea of taking a 2 1/2 20ga in to a magnum to me is ludicrous. It was a sweet little gun the way it was built. I would expect the same person who would lengthen the chamber of a 2 1/2 20ga would wax ad nauseum about the virtues of a 28ga.

410 not likely to fall under the 96 rule as it is, as you said, a bore and not a gage.

The rule was designed to suggest MINIMUM weights for comfortable shooting of a gun that fits you.

May I suggest that it was not the ribs that helped Darne guns but the type of closure. The Darne breech block closure eliminates headspace and thus recoil.

Regards,

No, pooch, I'm not a small-gauge guy. I have a Elsie 20 which is perfect for grouse for me but there's a time and place when my 20 SKB Model 500 o/u three-inch is also perfect, too. The time is as an older man. The place is slugging it all day in the muck for geese over tollers. A light gun is no burden although I've no desire for a 28 or .410.

Let me try this again. I follow the rule of 96 faithfully in restoring an old shotgun. My goal is to get back to where the gun was initially designed and believe an attempt to exceed that design limit will be my undoing.

Headspace is controlled by the depth of the rim seat versus the rim thickness of the shell. Either a break open or sliding breech gun can have normal or excessive head space. My 16ga Halifax which is basically an R type Darne which does not have the obturator discs except in the highest grade of the R series weighs in @ 2oz under 6Lb. This puts it those 2 oz shy of the 96 rule for an ounce of shot.
As I understand it the "Rule" was designed for full loads of black powder. It has been suggested when using smokeles, which has a lighter powder charge weight when loading to the same velocity the Rule can be altered to 90.
The fact that .410 is not a gauge has nothing at all to do with it. A normally loaded 2½" .410 could be fired comfortably from a 3Lb gun. It might not handle good but recoil would not be excesssive.
Altering the velocity of a load also of course alters the "Rule" as to appropriate shot weight. A 2 3/4 - 1¼ load would normally have less recoil than a 3¼ - 1 1/8 load from a 6 3/4lb gun based on the 96 rule for the 1 1/8oz shot because of its much lower velocity.
Burrard acrually had a much better rule than the 96, he recommended for sustained shooting, as in British driven shooting, to keep the recoil "Velocity" of the gun below 16FPS. While not an absolute A very close figure for this is obtained by multiplying the total ejecta weight (shot, wads & powder all converted to Lbs) by the muzzle velocity & then dividing by the guns weight. If this proves to be uncomfortable then just work to a lower figure than 16. This 16 rule will actually work out real close to the 96 rule for a given shot weight propelled at close to 1300 FPS.
As Smallbore said the "Only" thing the 96 rule applies to is Recoil & what is comfortable for one person may well be murderous to another.

I have said this before in at least one other thread. I don't understand the reluctance to establishing a US proof house - let me emphasize on a voluntary basis, i.e. not by government dictation.

There are so many safety issues that can be resolve by "proof testing."

Seems to me it would be a good "business opportunity."

A voluntary basis is the 'camel's nose under the tent.'

Please don't talk to me about anything involving A***s.

You know, we've gone round and round about the US Proof house discussions on this board. Seriously though, wouldn't it be only a moderately difficult lobbying task to get SAAMI to acknowledge proof and then to certify the means and the private companies permitted to prove arms? The SAAMI working group should be amiable to this if a bit of lobbying was done in this area.

Guys-
We already have US Voluntary proof houses. Each manufacturer proofs in accordance with their procedures.Because of US courts,the manufacturers have a vested interest in making sure each of their guns is safe. This is why we had to put up with heavy trigger pulls so long, and no US maker chambers for 5.6x50R. Things are bad enough with out getting the Gvt. any more into it. Mike

I understand the reluctance to establishing a US proof house, as others have noted, we don't need any more government oversight. Look at what the British and European governments have done on the business of gun ownership and regulation. As an American, I can't buy into it. To presume that it will remain voluntary is naive, IMO.

Guys, I'm not suggesting regulation. I'm suggesting a response to the suggested need. A SAAMI "authorized" proofer under some sort of SAAMI framework would be non-governmental but it would allow us to subject guns to proof based on reasonable standards and a credible entity. Not suggesting we need a new Obama cabinet post to handle the bureaucracy.

The British and many of the European proofhouses were around long before those countries adopted more restrictive firearms laws. It wasn't a case of using the proofhouses to protect the people from gun violence, but rather using them to protect the people from unsafe and potentially dangerous products. If you're going to take away all guns or similarly restrict private firearms ownership, then you don't really need a proofhouse.

Why would a (or several) private proof house evolve into some sort of anti-gun Trojan horse.Underwriters Labs has been around for eons and works just as intended. I am sure there are plenty of other comparable private standards organizations.

I didn't go back & check, but this discussion was up not long ago & went into multiple pages. The "KEY" word here is "Voluntary". As talk of a proof house arises virtually always the word "Mandatory" seems to creep in. This is what so many, including myself, object to. You cannot have mandatory without Government involvement.

It would have been helpful, prior to the standardization of US gunmakers on 2 3/4"/3" chambered guns, all of which--within a given gauge, except .410--have the same service pressure, if the industry had used proofmarks to indicate the standards under which their guns were proofed. There were no industry-wide standards prior to SAAMI (mid-20's), and even after that, we had a bunch of different chamber lengths for different gauges. And, from what I can find in the literature, different proof and service pressures. All of which wasn't much of a problem as long as appropriate factory ammo was readily available--which it was, at least up to WWII. After that, shorter American factory shells disappeared pretty quickly, and a lot of gunsmiths dealt with the problem by lengthening 2 1/2, 2 9/16, and 2 5/8" chambers to 2 3/4", and acting as if that solved the problem. Which it didn't then, and doesn't today on old American guns which have been rechambered--any more than taking a 2 1/2" Brit gun and punching it out to 2 3/4" means that you should use any and all American 2 3/4" factory loads in that gun, even if it's cleared 850 bar/"standard" CIP proof. The advantage of the British system is that we at least know when the gun has had its chambers lengthened, and that it has been reproofed. But that only happens because it's all mandatory over there.

Miller is right---again. Voluntary is key. I'm involved in forest sustainability and certification of private lands to the world's highest environmental standard: the Forest Stewardship Council. It's voluntary. It also provides greater access to more demanding markets. Your choice. Certification with independent third-party audits proves we're not Amazon North, as proofhouses "prove" specific standards have been met to assist safety and determine value. Your choice.

When was the last time anyone had their desk lamp "re-proofed" by UL after they changed the socket base or cord?

Where's Italiansxs when he's really needed? Can you imagine a proofhouse in the states in todays volitile gun law environment? Proof records could be abused. This shooting of a dozen people already has the gun control media making their own news about more gun control. Bob Shieffer already threw it out there this morning.

How many of you with a 90% Optimus or AHE or LC Deluxe or even a favorite family GH Parker or CE Fox would run down and have someone pump a couple near 20,000 psi rounds thru it and get out a big hammer and a metal stamp and beat on your pride and joy? Not many takers on that, I suspect.

+1. The high minded "voluntary" approach needs healthy measures of common sense and real politik. The back door initiatives (beaten down so far by an active pro-gun lobby) such as loading component and ammunition regulation and control come to mind. Keep government out, entirely, or we'll rue the day we went along with "voluntary" proofing.

Why would anyone lengthen a chamber when simply having forcing cones lengthened produce the desired result in most cases with probable better performance?

Erik,
I'd be guessing, but it's probably just uninformed pedestrians and gunsmiths. I shoot my damascus guns with 2 3/4" hulls loaded to low pressures in 2 5/8" chambers.

Erik, if "the desired result" is reducing pressure to what you'd get by firing a 2 3/4" shell in a 2 3/4" chamber, the only way to do that is with a 2 3/4" chamber. Sherman Bell's "Finding Out for Myself" tests showed that lengthening the cones usually resulted in some reduction in pressure. However, pressures still remained higher than when the shell was fired in a chamber of the appropriate length. It was usually only a question of a few hundred psi, but still higher.

Of course the tradeoff is that removing metal from the chamber (as opposed to just lengthening the cone, which removes very little metal) obviously weakens that area of the gun. That's why the Brits insist on reproof when chambers are lengthened.

If one begins measuring thwe wall thickness with an accurate gage, just forward of the rim seat & continues down the bbl the following will be found in a normal gun. The OD of the bbl decreases in diameter faster than the chamber dia so the wall will decrease until the end of chamber is come to. At this point the diameter of the cone decreases faster than the OD so the wall will reverse & begiin to increase until the end of cone is arrived at. Here the bore will remain essentially constant while the OD continues decreasing, so the wall will again begin to decrease. At some point the wall will become equal to that at the juncture of the chamber & cone. The cone can thus be lengthened to this point with no loss of minimum wall thickness. Even if the cone is lengthened more than this the point where the wall becomes thinner than that of the chamber/cone juncture will be moved down the bbl where pressure is dropping so no Danger point is ordinarily established. Thus the British ruling that a gun having a lengthened cone does not need re-proofing. On the other had if the chamber itself is lengthened the OD is still decreasing faster than the chamber so an immediate reduction of wall thickness occurs at the end of the chamber.

A couple of points though worthy of note, The point of maximum pressure is reached inside the chamer itself prior to this point of minimum wall so this does not in most cases actually represent a "Weak" point. Also most American guns were not built to a carefully stuidied out minimum as are some British game guns. Most American guns can actually have the chambers lengthened without encroaching upon the minimum required wall thickness.
Of far more real concern is the overall effect of using heavier loads than the entire gun was designed around. Also do not take these comments as a recommendation to rush out & have your chambers lengthened, I do not recommend such.
I a book by Roy Dunlap "Gunsmithing" copyrited 1950 he gives minimum chamber dimensions for the 2 3/4" 12ga. He doesn't specifically say so, but I assume these were then current SAAMI specs. Minimum chamber dia was given as .798" with a minimum cone of 5° per side. This calculates to a cone length of .400" to a bore diameter of .728". But, get this, he gave a minimum length from breech to beginning of the cone as 2.6136 or slightly shorter the 2 5/8" (2.625"). Using the minimum parameters the diameter of the cone at a point 2.750" from the breeech would be .024" smaller than the chamber or .774". This chamber was sanctioned for any & all 2 3/4" sheels in guns which were otherwise built for those loads.

I just saw an American made gun survive 'proof'

A friend, and very experienced handloader just fired off a major overcharge in a Ruger revolver. This is one of the new ones, in 357.

He was shooting 38 special cases, had a squib, and several rounds later found the rest of the powder in another load.

The gun barked a bit... and we had to drive the case out of the cylinder. This was very difficult. It was ironed in tight.

He went home and weighed the rest of the 3,000 loads he had on hand (yes, 3,000). All suspect ones were disassembled. No overcharges found.

I suspect ANY American made pistol will survive this kind of torture, and any of the few shotguns made here are just as stout.

We don't seem to have a big problem with US made firearms blowing up.

Back in the late '60s while shooting skeet in western OK I had the unpleasant experience of firing a hand load (someone else's load) that had apparently a double powder load in it. BIG bang and knocked me back against the low house. Everyone thought I had fired both barrels at the same time...which I had not. The barrel (left one) was bulged in the chamber area with the center of the bulge pretty much in line with the center of the chamber. I would not care to try that again!!

I recently saw the aftermath of a 20ga Fox ejector gun with the R chamber blown. Peeled away from the outside (where the pin on the ejector luggers runs through the frame) in towards the rib. That's the opposite of the way it happened with the first Parker Bell blew, which peeled out from the middle--starting from where the big pin on the extractor lugger runs underneath the rib. In both cases, the holes where those pins ride are obviously weak spots--although, as Bell's test showed, usually well capable of withstanding extreme overpressure loads.

A double charge may have been at fault in the case of the Fox. Reloads, and the shooter later found one or two with no powder.

Just my thoughts, but if creating a law with the intent of preventing gun failures were the goal, I would think that outlawing reloading would be the most effective.

I'm not saying I'm for it, just that most failures I've seen were related somehow to reloading.

There are failures caused by overloads, but most are tracked down to bore obstructions.Even failures in or near the chamber can result from obstructions, such as 20 ga shell in 12 ga or loose base wad from previous shell.
Mike

I don't recall the details of the actual load, perhaps someone here does. Some 10-12 yrs ago maybe longer, Buck Hamlin took a 12ga hammer L C Smith having welded bbls, don't recall now if they were twist or Damascus. Barrels were heavily pitted & considered very poor condition. He lengthened the chambers to 3½" & proceded to begin firing the heaviest loads of Blue Dot listed for 3½ shells in Hercules/Alliants manuals. As nothing unusual occured he began increasing the load & continued doing so until he just physically ran out of space in the hull, but never succeded in Blowing it up.
Note also the guns mentioned above which were blown up, Bells's test being at some 30K psi, none blew at the chamber/cone juncture, nor down the bbl.
It should not of course be supposed that in a well designed & made gun, using any normal loads that the wall between the chamber & extractor hole is a particular weak spot. Those which did blow at this point were not ordinary.

Miller, I recall reading those tests that Buck Hamlin did with the pitted L.C. Smith Damascus barrels in the LACA newsletter. I think he actually did eventually succeed in blowing up the barrels with what he called "Nuclear Loads", but it was only after reaching a point where the gun was blowing itself open at every shot. I'm not sure just how that occurred in a rotary bolted L. C. Smith, but the pressures had to be at least as high as Sherman Bell was reaching when his barrels finally let loose.

All the same, I think it should be a felony, punishable by hanging, to lengthen the chambers on vintage doubles. Just my opinion, but proper length shells are readily available, and modern magnum shotguns are just as available should the need arise. There is no need to turn nice original vintage cars into street rods either.

Keith;
Yes the LACA newsletter was where I read of Bucks experience. As I recall it though he didn't actually blow up anything but quit when the bbls started blowing open. I have probably heard of more guns with rotary bolts being blown open than any other bolting system. As I recall this was a big problem with some NIDs, I recall Elmer Keith encountered it with one.
As I understand it when the bbl hook puts enough upward pressure on the bolt it simply cams it around, the slope of the hook being greater than its coefficient of friction. Sort of like pushing down on a watermelon seed & squirting it out.

In the case of the blown Fox I saw, I would have initially voted for bore obstruction, probably a base wad, had the shooter not checked his reloads later and found a couple with no powder charge. That makes a double charge more likely, although it still could have been a base wad.

Re reloading, I'm aware of at least one modern double gun (from a "name" European maker) that blew from a factory shell. Inspection revealed that incorrect barrel boring was the problem, with a very thin spot (I don't recall for sure, but I seem to remember something closer to .010 than to .020) where it gave way.

I have been reloading target shotshells for over 50 years without a mishap, so far anyway
Given the bulk of such powders as Green Dot, Unique, and so on, if I have a double charge of 32 grains in a 12 ga hull I can't get hardly any shot in, so I see a problem right there.
Unlike pistol brass, too easy to put a double load in a 38 Special hull, that's why I charge the hulls and put them facing up in a block 50 at a time and shine a flashlight down to see they are all level with the powder before seating the bullets.
A big worry is to have no powder in the primer will blow the bullet halfway up the barrel = a bulge

MB in MB:
+1 and AMEN to your handgun reloading in units of 50 at a time, and over viewing all 50 cases to ensure each has powder. If one case does not have powder, your eye (MY eye, anyway) will IMMEDIATELY be drawn to the empty case - as it will not conform with the visual pattern. This powder check is an "unintended consequence" of using a single stage loader. This particular check doesn't exist with a progressive loader.
Just my $0.02, your mileage may vary.