The use of black powder and corrosive primers is a primary cause of pitting in some old shotgun barrels, especially if they were not cared for properly shortly after their use.  The results can be anything from slight to severe pitting and those pits effectively reduce the barrels thickness by the pits depth at that location.  If the shotgun bore is not mirror bright, a competent gunsmith will determine the depth of the pitting and take that into consideration when measuring barrel wall thickness for safety. 

With that said, here is a general 12 ga. guideline: 

In the forward 2/3 of the barrels length, a thickness of .025" has been suggested as a minimum thickness for safety reasons. The rear 1/3 length of the barrels, where the maximum pressures are exerted, requires thicker wall thickness.  The wall thickness at the juncture of the chambers and the forcing cone is the most critical and is where it has the highest pressure exerted. 

With some uncertainty, its assumed that a thickness of .090" at this juncture location would be about minimum. 

UK Working Standards recommended minimum wall thickness measured 18" from the barrel breech from Double Gun Classics p. 56, Vol. 1, No. 4 Jan-Feb, 2006:

2 1/2" 12g- .028
2 3/4" 12g- .032
Re-proof recommended minimum- .024

And, The Hunter's Encyclopedia from the German proof house: minimal wall thickness at end of chamber, regardless of length, for 12, 16 & 20 gauge guns should be 2.3mm (.0906") for 'ordinary good steel' or 2.1mm (.0827") if a 'Special Steel' was used. For the 24 & 28 gauges, due to their higher pressures, 2.4mm (.0945") was recommended.
Minimal wall of .6mm (.0236") was recommended in the "forward third" of the barrel.

There is greater pressure as the gauges go from 12 to 16 to 20 etc. therefore the minimum values might increase.

Learning the measurement of known original Parker barrels of the same gauge would be a good example to use to understand what Parker thought were safe values in the days when they were making shotgun.  The problem with this method is finding "original" Parker barrels and getting permission to take measurements.

The thickness of the barrel depends primarily on the frame size with respect to gauge, and secondarily on length. The barrels were finished by longitudinal hand filing to fit a general set of outside diameter checks. The filing was generally done to balance the gun at the hinge, but it could be muzzle heavy or light if the customer ordered. The longitudinal hand filing produced barrels that were not necessarily concentric with the bore, and wall thickness can vary side to side or top to bottom. There is no standard thickness.

Some guns appear to have thick barrels because full choke barrel muzzles are .040 or more thicker than the bore.

And, Some general observations made on the PGCA forum - 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.

Again, a word of caution - without a hands on inspection, these numbers mean little; don't risk life and limb shooting bad barrels.  Have them inspected by a competent and knowledgeable gunsmith before shooting an older gun of unknown history or condition.

Here is a link to a  video that demonstrates how barrel wall thickness is tested.

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