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ThreeDogs

Minimum walk thickness?

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bamboozler

Everyone is entitled to their own opinion, but not everyone is entitled to their own facts.

 

Here's what one poster wrote that has actually measured the wall thickness of many guns.  This was posted on the PGCA Forum -- it's a very interesting read:

"I don't chime in on these conversations often, but the topic keeps coming up. Many know that I own Vintage Firearms, Inc. and buy and sell many Parker guns. Every gun I buy has been measured for minimum barrel wall thickness. In addition, I measure many more sets for others or when contemplating guns to buy for inventory. I measure 300-500 sets of barrels a year. Not all Parkers mind you, but many are. For all I have had through my shop, I have records in spread sheet format that allows me easily to go down a column to see how the numbers fall.

Regarding accepted barrel wall thickness for shooting modern loads, it seems everybody has an opinion. Many of the gunsmiths that render opinions on wall thickness of 25 thou or above work more on modern guns than vintage doubles. If you ask a person who has spent his life working on fine VINTAGE double shotguns, they better understand how these guns were made and have much different opinions on the subject. Most all agree that even 18 thou in the forward half of the barrels is not dangerous from a bursting or bulging stand point, but rather the risk of dents and damage that cannot be repaired as there is not enough metal to work with. More on 18 thou later.

If you ask me, the Brits have been the fussiest about barrels for almost 2centuries, and set the mark for proof testing. Forget opinions based upon everybody bloviating and regurgitating what they have heard or what their friend's opinion is. The British proof houses regularly try and blow up perfectly good guns! They see what barrels can handle by passing not one, but two definitive proof loads through each barrel. I believe the loads are 18,500psi. We all shoot loads that are below 12,500psi (magnum loads), and most of us shoot more reasonable loads that run under 10,000psi. And the guys that have patterned their guns with loads like RST Shells recognize it is not speed that kills, but the nice even patterns premium ammunition provide. RST Shells don't exceed 8000psi. Go to www.rstshells.com for very affordable, safe loads for your beloved doubles.

If barrels with 20 thou wall thickness were regularly failing the proof they would not mention that as the recommended minimum. The facts are that barrels under 20 thou regularly pass proof and are deemed safe. Barrels with 20 thou and all other characteristicds in good shape pass proof in overwhelmingly high numbers. It is extremely rare for failure in the rigid proof testing for barrels in excellent condition because of wall thickness of 18 thou or above. Barrels fail for other reasons, but not often from bursting or changing bore diameters as in bulges. AND REMEMBER, THIS IS WITH 18,500 PSI LOADS!

Doesn't repeated, large sample, empirical testing that occurs in very controlled circumstances trump untried opinions? Especially with DEFINITIVE PROOF LOADS?

The British Gun Trade Association clearly states that 20 thou is the generally accepted minimum for judging healthy guns. This is stated in the Jan/Feb 2012 issue of Shooting Sportsman, and in another Shooting Sportsman article from Sept/Oct 2009 issue. Unless you are shooting guns with obstructions in the barrels, there is no measurable risk shooting reasonable loads in guns with 20 thou wall thickness at least 15" from the muzzle, provided all other issues are sound, like tight ribs, no serious dents, etc. This is not opinion, but data collected for decades under controlled testing.

Now, from my experience I think many Parker collectors and shooters may be surprised that I have measured at least 20 guns that were 20ga. or 16ga. guns on "O" frames that were definitively factory original in the way of blue and bore diameter. They were never backbored or polished inside, and never filed or machined on the outside since leaving the factory, and they had areas 6-12" back from the muzzle that were 18 thou, FROM THE FACTORY. This is almost always in a 3-4" area very close to the top rib or bottom rib, and only on one side of the tube. As one person on this thread mentioned, virtually all vintage American doubles have a very noticeable lack of concentricity, ie thicker on one side than the other. There is also the matter of soldering on the ribs, with the required filing of overflow solder tight in to the rib, creating these thin spots. 

All of these guns were very lightweight Parkers. Where most 20ga. Parkers weigh in the neighborhood of 6 1/4lbs., often a few ounces more, how do you think the factory came up with the guns that weigh less than 6lbs. or even 5 3/4lbs.? It is damn hard to hawg a butt and remove 3 oz. of wood. It is usually more like 2 ounces with a lot of hawging. And once hawged out, how do you think they keep the gun from being barrel heavy? They filed metal from the barrels, that is how. 

The guns I refer to have been shot for generations, and 10 years ago hunters would regularly use high base shells for everything. After 70-100 years of use they are still unchanged and have perfect barrels. 

How many of the opinion makers actually own high quality barrel wall thickness gauges? Not many I can assure you. I travel the country and am surprised how few buyers of fine shotguns own one, let alone know how they are used. If you are going to buy more than a few shotguns in your life, I recommend looking at the Hosford and Co. barrel wall thickness gauge. One mistake in buying a bad set of barrels on an expensive gun will pay for the gauge 5 times over. The Hosford gauge is very convenient and portable. Either that or rely on someone that has one before finalizing any deal in which the wall thickness is not guaranteed by the seller. Just my opinion on that. No dog in the race.

Just examing 100 Parkers I have sold over the last 3 years, 24 of them had at least one of the two barrels with wall thickness under 23 thou. Without a lot of researching each individual gun, I can generally say I don't buy or sell any guns you all would consider unworthy at least as a sound shooter, and in general I have above average shotguns. THAT IS 25% OF THIS SAMPLING THAT PROSPECTIVE PARKER OWNERS WOULD DISMISS IF THEY WERE FOLLOWING THE 25 THOU RULE. 

I guess my point is this: It seems there are quite a few folks that come to this forum as being inexperienced with buying and shooting Parkers, and are looking for sound info from members to utilize in getting started shooting and collecting Parkers. Rather than use guesstimates and opinions, why not recognize the results of strict empirical data gathered over the decades of testing provided to us by the Brits? It is certainly better than having fellows looking for a light weight Parker for the uplands simply give up because they can't find one with both tubes over 25 or even 30 thou, as some have stated in this thread. Many of those following that advice would pass over some very fine Parkers for no reason at all. And then they would have to buy Fox guns, as most of them are much lighter than like Parkers! We don't want that do we?

And again, most of the members of our association have no clue what their Parker's barrel wall thickness is. MANY of them are shooting guns that are under 25 thou, and have for decades. 

A TOPIC FOR THE NEXT THREAD...CAN SOMEONE FIND THE PRESSURE CURVE DIAGRAM SHOWING HOW A SHOTGUN SHELL DEVELOPS PRESSURE UPON DETONATION, AND AS IT TRAVELS DOWN THE BARRELS UNTIL FINALLY LEAVING THE MUZZLE. I saw that diagram years ago, and it gives a graphic demonstration as to why barrels that are on the thin side forward of the midway point are not at risk of bursting. The entire pressure spike upon detonation occurs in the first 13" or so from the breech, and then is like a pussy cat going down the balance of the barrels. That would help folks understand the physics and mechanics involved.

Sorry for the long post. It is my reason for not writing. I can't say things in a few words while typing!"

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KDGJ
16 hours ago, ThreeDogs said:

So I am getting succored in to an older British double......what kind of minimum wall thickness do I need to look for to be safe with modern ammo. 2.5 inch chambered gun and I see no reason the lengthen the chambers.

 

thanks in advance!

 

Just to be clear, are you looking to shoot modern 2 3/4" American ammo in a 2 1/2" Brit gun?  If this is what you want to do, then you need to understand the pressure of the load also.   These guns aren't proofed for modern SAAMI pressures.

 

A fairly recognized expert, on British guns, views of barrel wall thickness DIg Hadoke MWT.

Ken

 

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salmontogue
36 minutes ago, lee sykes said:

 I was loosely quoting McIntosh.  It is true that 9" from the breech is where most shotgun barrels fail.   Not sure why but it is generally considered to be true.   Probably the point where the gradual reduction in thickness from the chamber area becomes critical. 

 

Charles Askins, Elmer Keith and Townsend Whelen made similar comments.  Askins described it as the "worst possible place" because it is close to the shooters face and hands.  I agree.

 

Perk

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oak stob

I recall the 9”-ish range being offered as a burst zone more than the 1 1/2” range....I always thought that was due to where the barrel taper work began in earnest rather than representing a peak pressure area...a combination leading to worst case.

 

At certain times, the Unanswered Prayers song of Grath Brooks comes to mind.

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salmontogue

Modern manufacturing processes would suggest superior metallurgy and significantly more consistent and accurate machining of barrels.  I'm not suggesting the older guns are deficient but I am suggesting that the state of the art has developed with the passage of years.  CNC, air gauging, laser measurement, electronic inspection (fluoroscopy and electron microscopy) and higher strength materials have all changed manufacturing.  Adding weights to improve balance and choke changes are one thing, removing barrel material through back-boring is something else entirely on an older gun. 

 

Engine blocks, considered for rebuilding, are routinely subjected to fluoroscopy and/or Ultrasonography.  I doubt many gunsmiths have this equipment, or do they?

 

Another consideration....where has an individual gun been during its lifetime, has it been misused, has it been stressed with improper loads, has it had repairs not immediately obvious and are there troublesome imperfections?

 

Perk

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Larry Brown

Re the long quote above by Bamboozler:

 

I'd say that all countries that proof test their guns are pretty fussy about safety, and are using a government proofhouse to try to protect those who purchase guns.  Most of them are now using the same proof standards, codified under an organization called CIP--so British proof = Spanish proof= German proof etc.  (Although they still all use different proofmarks, the better to confuse Americans.)  That being said, American gun manufacturers also proof test their guns, and to essentially the same standard (18-19,000 psi range).  The only difference in this country is that it's done by the gun industry and not by a government proofhouse.  But obviously, gun manufacturers are the ones who are going to suffer if their guns self-destruct in the hands of gun owners, taking various bits and pieces of the owner along with the bits and pieces of the barrel.

 

However, his reference to proof testing overseas is misleading.  On MODERN British guns, more of them will be proof tested with high pressure loads than was previously the case (when most of them were made with 2 1/2" chambers . . . in other words, the bulk of the used British guns we end up buying.)  Those guns with the higher proof, if made today, would carry the proofmark SUP (for superior, or magnum proof)--and would have 2 3/4" or perhaps even 3" chambers.  Previously, those guns would have carried the 1200 bar proofmark.  For older British guns, and for those still proofed at lower pressures, they will today carry the proofmark STD (for standard).  As noted, if they're older, they would almost certainly have left the factory with 2 1/2" chambers, and would carry the proofmark 3 tons, or later 850 bar.  The proof pressure on those guns equivalent to the Parkers etc to which he refers would have been 14,000 psi--not 18,500 psi.  So you can see why it is we don't want to run 12,000 psi loads through those guns:  they come relatively close to the pressure at which the guns were proofed.  But you do have plenty of safety cushion if you shoot loads in the 8,000 psi range, or less.  I've shot 1 1/8 oz reloads at pheasants through those guns, at a pressure not much over 7,000 psi, with velocity in the mid-1100 fps range.  I wouldn't want to shoot a round of trap or skeet with those in a 6 1/4# British game gun.  But you don't shoot that many shells at pheasants (unless you're shooting driven birds), and my experience is that a few shots with them are usually unpleasant only to the pheasants.

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Chukarman

My gunsmith uses ultrasound to measure barrel wall thickness. Every two inches from the forcing cone to the muzzle - including chokes. This , IMO, provides the definitive barrel wall thickness measurement. The traditional measuring devices can yield varying results depending on the tool, method, and experience of the tool user. An example -- when purchasing a high end, lightweight Brit 'best' 12 bore recently, the maker provided MWTs at .022" and .021". The ultrasound showed the MWT of .025 and .028. This on a 6 lb. 3 oz. gun with 27" barrels - pretty light gun for a 2-1/2" 12 bore.

 

The key Brit standard for MWT is measured 9" from the barrel breech. As pointed out above, passing the British proof test provides not only a safe margin, but a base measurement for checking whether a gun's barrels are in or out of proof.

 

 

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Dave in Maine

The "worst possible place" of 9 inches downstream has to do both with the placement of hands, fingers, faces and eyes and with the way barrels are profiled.  More or less uniformly, shotgun barrels have rather substantial wall thickness at the chamber end.  The wall thickness then decreases to a minimum somewhere past mid-barrel, then increases somewhat near the muzzle.  The point 9 inches downstream is not necessarily where the highest pressure occurs.  As noted, that highest pressure is in the chamber.  Rather, the 9 inches is in the neighborhood of the highest (hoop) stress on the metal.  Stress is the amount of force (caused by the pressure) per unit of cross-sectional area (wall thickness).  So, while at the chamber end there is the maximum pressure, there's also the maximum wall thickness there (often as much as .125, i.e., one-eighth of an inch). 

 

Let me state an example.  Note that the numbers I'll use are merely an index of how the metal is stressed and not actual engineering values.  I'll cal it "relative stress".  I am leaving out of the example various engineering constants which would wash out in the comparison.  Also note that the numbers I'll use are not necessarily actual numbers but rather have been chosen for the purpose of illustration.

 

Assume a (max) load generating around 12,500 psi at the chamber and wall thickness of 0.125 inches, also at the chamber.  Divide pressure by wall thickness to get "relative stress":  you'd get a number around 100,000.  Going down the barrel to a point where the pressure has diminished to 2,000 psi, and the wall thickness of 0.020 inches.  Divide pressure by wall thickness to get "relative stress" and you get a number around ... 100,000 again.  Same relative stress.

 

But the wall thicknesses of shotgun barrels do not vary in a linear fashion.  The shape and thickness of barrels (particularly in sxs guns) is a complicated curve dictated by things like profiling them for weight distribution (so the gun balances and swings properly), the size, shape, weight and placement of ribs (many French guns have only one rib or even no rib except at breech and muzzle, to save weight and modify balance and swing), regulation, and so on.  The "why" for that non-linear variation is not so important as recognizing it exists.  FWIW, barrels get thicker at the muzzle end for several reasons:  to protect them against damage, to provide enough meat for both regulation and choke boring, to modify the swinging characteristics.  From an engineering POV interested in stress, the muzzles have to be relatively thicker because the end of the barrel creates a free surface where the stress of the projectile passing through, stretching the steel in its passage, can only be distributed in one direction (upstream) and not in both directions as elsewhere in the barrel.  Ever wonder why old cannons have that exaggerated rim at the muzzle end?  The need to distribute that stress is why - early cannons had a tendency to burst and banana-peel there.

 

Getting back to my example on "relative stress", the point 9 inches downstream is possibly the point of highest relative stress, or at least is close enough such that it's a useful place to measure.  At 9 inches downstream the barrel wall thickness has decreased but the pressure may not have decreased to the same degree.  Assume that at 9 inches the pressure from that max load (12,500 psi at the chamber) has decreased to 4,000 psi.  Assume further the wall thickness is now 0.030 inches.  (Still a rather substantial wall thickness - see the Parker specialist's note quoted above.)  Doing the division again, we find a relative stress of 133,333, a full third higher than at the chamber or at the point where the wall thickness was .020.  The relative stress on the metal at the 9-inch point is likely the highest relative stress anywhere in the barrel, and that's why a "good" measurement there is so important.

 

I hope this helps.

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ThreeDogs
21 hours ago, KDGJ said:

 

Just to be clear, are you looking to shoot modern 2 3/4" American ammo in a 2 1/2" Brit gun?  If this is what you want to do, 

 

 

I never suggested that was what I wanted to do, I’m happy shooting 2.5 inch rounds I see no reason to lengthen the cone for modern ammo.

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Chukarman
On 3/12/2018 at 8:52 AM, KDGJ said:

 

Just to be clear, are you looking to shoot modern 2 3/4" American ammo in a 2 1/2" Brit gun?  If this is what you want to do, then you need to understand the pressure of the load also.   These guns aren't proofed for modern SAAMI pressures.

 

A fairly recognized expert, on British guns, views of barrel wall thickness DIg Hadoke MWT.

Ken

 

 

To be fair it needs to be understood that American and Brit makers pressure test using different systems/standards.  Pressure measurements are inferred from measuring the length of lead cylinders (yoelding by inference Lead Units of Pressure or LUP) that are 'crushed' in a device that is attached to a test barrel. Testing rifle pressures uses copper cylinders and yield Copper Units of Pressure (CUP). This is the basic method traditionally used prior to the invention and application of pressure reading strain gauges. Unfortunately the Brits do not use the same exact standards and equipment as makers in the USA. Also, be aware that a British ton is 240 lbs. more that a US (short) ton, so a Brit (imperial) ton = 1.12 US tons. This is the short version. Whole books are written about this.

 

Personally, I use 2-3/4" Winchester AA hulls in loading for my 12 and 16 gauge short chambered guns... fired Brit hulls I've measured are usually just over 1/16" shorter than standard WInchester 2-3/4" hulls and, provided that the pressures obtained in these loads is within the proof range, I see no reason not to do so. I am not recommending this, nor am I trying to provoke an argument. Just sayin'...

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lee sykes
1 hour ago, Chukarman said:

 

To be fair it needs to be understood that American and Brit makers pressure test using different systems/standards.  Pressure measurements are inferred from measuring the length of lead cylinders (yoelding by inference Lead Units of Pressure or LUP) that are 'crushed' in a device that is attached to a test barrel. Testing rifle pressures uses copper cylinders and yield Copper Units of Pressure (CUP). This is the basic method traditionally used prior to the invention and application of pressure reading strain gauges. Unfortunately the Brits do not use the same exact standards and equipment as makers in the USA. Also, be aware that a British ton is 240 lbs. more that a US (short) ton, so a Brit (imperial) ton = 1.12 US tons. This is the short version. Whole books are written about this.

 

Personally, I use 2-3/4" Winchester AA hulls in loading for my 12 and 16 gauge short chambered guns... fired Brit hulls I've measured are usually just over 1/16" shorter than standard WInchester 2-3/4" hulls and, provided that the pressures obtained in these loads is within the proof range, I see no reason not to do so. I am not recommending this, nor am I trying to provoke an argument. Just sayin'...

I have done the same.. Upon comparing fired case lengths,. I came to the same conclusion.. Geoffrey Bothroyd wrote in his book that I no longer have (shotguns and cartridges?). That plastic cases with their thinner bodies were less likely to raise pressure in a short chamber than the old paper hulls.  There are exceptions.  I once had an L.C. Smith that bit off the ends of fired hulls like they were cigars.  I had the chambers and forcing cones lengthened and that fixed that.  The one 2 1/2" Brit gun I owned was quite happy with 2 3/4" plastic.  

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Larry Brown

Agree with Cman and Lee above on reloading.  2 3/4" hulls work well in MOST short-chambered guns--the exception being some made (mostly prior to 1900) with very sharp and steeply tapered forcing cones.  In those, you'll find that you need to use true 2 1/2" shells.  But otherwise, if you watch your pressure--and that's particularly easy to do when reloading for 12's--the longer hulls will not cause a problem.  If there is an issue, you'll likely know because you'll have blown ends on your ejected hulls.

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