doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com What is considered safe min wall thickness?

I am looking at a 100 year old 16 gauge sxs that has a minimum wall thickness of .023 left and .024 right. Is this considered safe to shoot?

Thanks

That's getting pretty thin to be safe IMHO. My personal minimum is .025 with clean bores. I understand the Brit proof houses will accept down to .020 for proof. If your gun has pitting in the bores that further reduces chances for being a safe shooter. You may decide to go ahead with low pressure shells, but it has to be a personal decision. I'd recommend finding a gun with more sound barrels...Geo

I would be shooting RST shells exlusively as the barrels are 2.5"

20 thou used to be the minimum that they would put through proof. Unless it's a 20 or a 28, it should be fine.

Question 1-9 is WHERE is that .023"??

Please see
http://www.lcsmith.org/faq/thickness.html

If safe now you understand that you can never have it honed further? It is thinner than I am comfortable. Not as much for pressure because if you reload you can load reasonably low pressure shells. I worry more about dings and ease of denting thin barrels. Also where is the thin area. Near the muzzle is fine but ten inches in front of the chambers not so much.

old ed likes no less .030 7 to 9" down from the muzzles and no less than .090 in front of the chambers.

even that sounds kinda thin to me. i mean, look at a mic set to .090. do you feel comfortable with an explosion going off in front of your face with only that thin metal between you and the fire?

and from drews link:

"Every vintage shotgun, and shotgun barrel, should be evaluated before use by someone with the equipment and expertise to properly do so, and then shot only using ammunition with ballistics similar to that for which the gun was originally intended."

amen to dat!

No, that's not quite correct. They (Birmingham and London Proof Houses) will inform their customers if the minimum barrel wall thickness falls below .020" even though it may have passed proof with thinner than .020" MWT. That information is direct from the chief inspector of the Birmingham Proof House.

I am being told by some that english guns are a different breed and that they are generally very thin.. Is this the case? the gun I am referring to is a holland.

Thin compared to gun made in the US? Yes. US made doubles are much heavier, thicker in the barrels if you want to call them that.

You do understand that pressures tend to be higher in smaller gauges do you not? Anything under .025 is thin in my books. If you are going to shoot only CIP loads or RST loads intended to keep pressure down you may be safe but with a small margin. If you are going to shoot US made, standard off the shelf factory loads, with pressure much higher than CIP then no it is not safe to shoot. The world is full of thin barreled gun which are borderline safe or not safe. What is your level of risk tolerance and what are you risking if wrong? Money or flesh?

Many (most?) English guns have been designed for light weight and fast handling. The barrel walls are made thin where pressure allows to reduce weight and improve balance and handling. With suitable English loads (which tend to be lighter and lower pressure and shorter cases than many American loads) they are as safe as any gun. Wall thickness requirement to survive the pressure varies down the barrel - peak being near the front of the chambers, and being much lower at the muzzles. Generally - barrels can be very thin at the muzzles from a pressure point of view, but would dent very easily.
20 thou towards the muzzles would be considered adequate, but will dent easily. However, much more is needed nearer the chambers. It is impossible to put an exact figure on this and expert advice should be sought if there is any doubt - and any advice given, particularly in respect of cartridge type and performance should be followed carefully.

Where an English gun is 'in proof' (i.e. not excessively enlarged in bore, pitted, altered or damaged since the previous proof) it will normally be safe FOR THOSE LOADS AND CASES FOR WHICH IT WAS PROVED. For many English guns that will be 2 1/2" cases and light (by American standards) loads.

British guns are all subjected to proof to see if they can withstand two charges in each barrel that are 25% higher than the maximum service pressure of CIP commercially produced ammo (which typically has a 15 - 20% margin of safety built in.

The thinnest safe barrel depends on what it is made from. Good steel will hold up better than poor quality steel of the same thickness etc.

If the gun is in proof and you use the right ammo, it will be fine.

I have seen guns made by the best British makers as lightweight 12-bores with original barrel minimums of 18 thou.

We in the Trade usually consider 20 thou a minimum for commercial purposes, which is why you see so many thin walled gun in auctions.

I have seen guns with damascus barres pass proof with a minimum of 7 thou.

I have also seen guns with thicker barrels fail proof.

Uniformly thin barrels will hold up better than thick barrels with steep gradients to thin spots.

There is no minimum wall thickness for proof testing. If the barrel withstands the test without material change, it will pass.

Dig, what are the reasons that a gun having a MWT of .007" would even be submitted for proof? Just to prove it could pass?

SRH

Guns should of course be used with shells which meet the criteria for which they were designed. This predominately though has to do with factors other than the minimum thickness of the barrel wall at some point down the barrel.
"Low" pressure shells is a term we have applied to shells having a lower Max "Chamber" pressure. This does not automatically equate to a lower pressure down the rest of the barrel. If two different loads push the same amount of shot out the barrel at the same velocity with one having say 10 K PSI & the other 8K PSI then those 2K PSI's have to be made up somewhere. They would thus be expected to be spread out over the forward portion of the bore thus would show a very slight "Increase" in pressure once about 4" from the breech has been passed. This is not to be construed as saying these low pressure loads are unsafe in a barrel having this wall down the barrel, but it also does not make them particularly safer on that account.
Total pressure can essentially be lowered by only one method & that is to lower the overall ballistics of the load, IE lighter shot charge, lower velocity or both. Max pressure can be manipulated by powder burn rate & a few other minor factors but as stated what you gain in one area you lose in another.

Miller, I think you either misstated something or else I'm not understanding. I've never seen a pressure curve that shows pressure increasing at any point down the bore. What does happen is that with some loads, the pressure drops off faster than with others. I can accept that your theoretical 8K load might have decreased less at 4" from the breech, relative to its initial pressure--that is, a pressure curve that does not decline as sharply--compared to the 10K load. But I'd have to see the results of a test in which the pressure increases, say, from the end of the chamber to the 3" point, then begins to decrease again.

Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, 1897
http://books.google.com/books?id=inQCAAAAYAAJ&source=gbs_navlinks_s

�Powder Pressures At Different Parts of the Gunbarrel, 1895� p. 32
http://books.google.com/books?id=inQCAAA...ure&f=false
(scroll down and click on p. 31)

The choice of 32 grains of Ballistite is confusing, as 24 grains is the published load for a 3 Dram equivalent; 26 grains for 3 1/4 Dram.
Alfred Nobel's 'Ballistite' was introduced in 1887, patented in 1888, then �Sporting Ballistite� (for shotgun shells) was patented in 1889, but not released to the trade by Nobel�s Explosive Co. until 1895. The published summary of the Trial states �Ballistite� rather than �Sporting Ballistite�.



1 1/8 oz. #6 shot at 1220 fps
Using the estimated Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch) (Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI
Note: Modern transducer measurements average 500-1000 higher than LUP

32 Gr Ballistite Dense (1 1/8 oz. at 1232 fps = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624psi)
2 1/2" - 2.2 Tons = 6272 psi
6� - 1.2 Tons = 3718 psi
12� - .81 Tons = 1602 psi
18� - .33 Tons
24� - .21 Tons

42 Gr Schultze Bulk 1 1/8 oz. at 1220
Breech - 1.69 Tons = 4558 psi
2 1/2� - 1.49 Tons = 3886 psi
6� - 1.36 Tons = 3450 psi
12� - .92 Tons = 1971 psi
18� - .55 Tons = 1178 psi
24� - .32 Tons

83 Gr No. 4 Black Powder at 1222 fps
Breech - 2.26 Tons = 6474 psi
2 1/2" - 1.96 Tons = 5466 psi
6� - 1.26 Tons = 3114 psi
12� - .9 Tons = 1904 psi
18� - .37 Tons
24� - .27 Tons

An early morning (at least here in the desert ) rant

There is a very disturbing trend among dealers and auction sites (look through Julia's listings for this month) to post meaningless MWT numbers. WHERE is that MWT and how about the chamber, forcing cones, and the critical section from breech to 9"??
Measuring WT properly is hard, and takes the equipment, time (which is money), experience, and INTEREST to be done right. I've found some unexpected and frightening numbers, and those of us with an interest in vintage guns really have no excuse today for not measuring, or finding someone to measure for us, wall thickness on guns we intend to use.

What is your recommendation for this? The Hosford device that Kirby sells?

Gartenguy: an unbelievably talented and generous friend that is part of this Forum made me a Manson-style gauge. Works great and quite consistent. My only improvement was to affix a tape measure and make the pin slightly more 'pointy' in order to get in the bottom of pits.
It is very helpful to have an assistant help rotate the barrels which I affix to a weighted box (in which I carry the gauge and bore gauge) using bungie cords, and record numbers.







http://www.brownells.com/gunsmith-tools-...-prod20463.aspx

What are the pitfalls of using a Skeets bore gauge for the inside measurement and a micrometer for the external measurement and just doing the math for the wall thickness where depth of pits are not needed?

I am sure others will chime in, but one issue is that the bore may not be concentric with the outside of the barrel.

Not just not concentric, but the inner bore, and outer wall will not be precision circles in cross section. We are talking mostly hand made guns here, the barrels hand filed and fitted by a barrel maker, who may or may not be a master of his art. Then factor in older guns, which may have been subjected to barrel repairs, bore honing etc. that may or may not have been carried to by a qualified, and competent smith. There are a myriad of conditions that will lead to localized wall thickness variation.
The current tools used to measure wall thickness aren't perfect, but are very affordable, and with patience and skill can give a good indication of wall thickness. But it's always prudent to have a competent SXS specialist gunsmith confirm your findings!

Larry;
Either I totally goofed in what I said or you misread it badly. Certainly after the max peak is reached the pressure begins to fall & does so continuously till the load exits the muzzle. The lower & higher were meant to be in comparison to the other load. If two loads give the same ballistics at different pressures the one which began with the lowest max chamber pressure will have the highest barrel pressure. Exact curves will of course vary with different loads, but about 4" from the breech is the general vicinity where the two curves will cross.
Thus to use the same two examples I used before if the MWT is out around 18"-24" from the breech the load which had 8K max chamber pressure will have higher pressure out at the MWT area than will the load which started with 10K max chamber pressure. Certainly in both cases the pressure Out there is going to be much lower than the max chamber pressure, it don't increase anywhere after th peak unless it hits an obstruction.
The point is though assuming a sound gun with ample wall thickness in the breech area with a thin area Down the barrel a ways you are not doing it any favor by lowering the chamber pressure if you do so without reducing overall ballistics of the load.
Lets say for example again that the proof of the gun is good for a load giving that 10k PSI max pressure. Say you have a load which pushes 1oz shot out a 1200 FPS at that pressure. That load will give the lowest pressure "Down There at the Thin Spot" than any load pushing that same 1oz of shot out @ 1200 FPS but with less chamber pressure than 10K PSI.

OK. Got you now. Thought you were talking about a pressure increase somewhere along the curve. Comparison between two loads of different peak pressures: we're on the same sheet of music.

This question comes up so often, and this thread will again if someone uses the 'Search' function, so thought I'd post this template I use when measuring barrels. The format changed a little from the Word doc but it might help those with gauges

Date_____________________________________

Maker___________________________________

SN______________________________________

...................................LEFT...........................RIGHT

Chamber_______________________________ ____________________________________

Bore __________________________________ ____________________________________

Choke _________________________________ ____________________________________

_________________________________ ____________________________________

Choke Length___________________________ ____________________________________



WALL THICKNESS

End of chamber ___________________________ ____________________________________
(.105�)
Forcing cone ___________________________ ____________________________________
(.100�)
9� from breech____________________________ ____________________________________
(.040�)
9� from muzzle____________________________ ____________________________________
(.025�)
MWT_________________@_________________ _____________________@_____________

Comments:

Unfortunately, when I first started measuring for guys, I gave them the sheet and didn't keep a summary but am doing so now. Here are previously posted results:

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=224090&page=all

Fox A grade 16ga, 26" barrels, 2-7/16" chamber, 0.085" and 0.086" minimum wall thickness at the end of the chambers, .734" and .734" diameter at the end of the chambers, 5/8" long forcing cones, #4 barrels, gun weighs 5lbs 9oz, serial 302XXX.

Parker VH O Frame 16ga, 28" steel barrels, serial 134XXX, 2-9/16" chamber, .082" and 0.085" minimum wall thickness at the end of the chambers, 0.730" and 0.732" end chamber diameters, 3lbs 2oz unstruck barrel weight, gun weighs 5lbs 14oz.

Fox XE 16ga, 30" barrels, serial 301xxx, .072" and .080" minimum wall at chamber ends, 2-7/16" chambers, .733" and .733" end chamber diameters, gun weighs 6lbs 3oz

Parker DHE 16ga, 32" steel barrels, serial 212XXX, 0.090" and 0.092" minimum wall thickness at chamber ends, 2-9/16" chambers, .735" and .736" diameter at chamber ends, barrel unstruck barrel weight 3lbs 11oz, gun weighs 7lbs 4oz.

Lefever H grade 16 gauge, 28" twist barrels, .090" and .092" minimum wall thickness at chamber ends, 2-5/8" chambers, .729" and .728" diameter at chamber ends. Gun weighs 6lbs 4oz

Ithaca Flues Grade 4E 16 gauge, 28" Krupp fluid steel barrels, 0.091" and 0.092" minimum wall thickness at the end of 2-5/8" chambers, daimeters of .728" and .730" at the end of the chambers, gun weighs 6lbs 6ounces.

Parker DH 12 bore, 28" Damascus barrels, 1 frame, .086" and .090" minimum wall thickness at end of the 2-5/8" chambers, gun weighs 6lbs 14oz. Serial #84XXX, My Skeets gauges are in capable of measuring the diameters at the end of the chamber.

Bissel Birmingham boxlock, 20 gauge, 25" steel barrels, made in the 1920s, .084" and .092" minimum wall thickness at the chamber ends, 2-1/2" chambers.

AYA #1 20 bore sidelock, factory 2-3/4" (70mm) chambers, 30" barrels, built and proofed in 2006, .092" and .091" minimum wall thickness at the end of the chambers.


Minimum chamber wall thickness
http://parkerguns.org/forums/showthread....ents&page=4

English:
James Purdey (1898) 12b., 2.5" chambers: 0.100.
James Woodward (1909) 12b., 2.5" chambers: 0.098
James Woodward (1909) 12b., 2.5" chambers: 0.103
Boss (1897-8) 12b., 2.5" chambers: 0.090
Westley Richards 12b., 2.5" chambers: 0.090
James MacNaughton (1895) 12b., 2.5" chambers: 0.095
Wm. Pape (1898) 12b., 2.75" chambers: 0.100
EM Reilly (1887-1904) 12b., 2.5" chambers: 0.091
Henri Egg (1870) 12b., 2.5" chambers: 0.108
WH Monks (1875-87) 12b., 2.5" chambers: 0.116
WC Scott (1905-6) 16b., 2.75" chambers: 0.105
Francotte (1938) 12b., 2 5/8" chambers: 0.098
Westley Richards (1905) 20b., 2.5" chambers: 0.098
WW Greener (1922) 20b., 2.75" chambers: 0.098

Belgian:
Francotte (1894-5) 12b., 2.5" chambers: 0.085
Francotte (1896) 12b., 2.5" chambers: 0.085
Francotte (1930) 20b., 2.5" chambers: 0.095

French:
Verney Carron (1950s) 12b., 2 5/8" chambers: 0.100

German:
Wilhelm Brenneke (1902) 12b., 2.5" chambers: 0.110

Some of the 12g for which I have numbers

12g 1913 Sterlingworth
Chamber & forcing cone lengthened and honed by Briley
End of chamber L .107 / R .109
End forcing cone L .097 / R .102
9� from breech L .040 / R .045
9� from muzzle L .030 / R .033

12g 1903 Smith 00
End of chamber L .110 / R .107
End forcing cone L . / R .
9� from breech L .048 / R .049
9� from muzzle L .035 / R .035
MWT L .031 3� from muzzle / R .027� 7� to 5� from muzzle

12g 1906 Smith 00
Bore .730 and possibly honed. Chamber and FC had been lengthened.
End of chamber L .112 / R .118
End forcing cone L .111 / R .118
9� from breech L .044 / R .053
9� from muzzle L .027 / R .034
MWT L .025 10� from muzzle / R .034

12g 1929 Smith Field
End of chamber L .095 / R .101
End forcing cone L . / R .
9� from breech L .043 / R .045
9� from muzzle L .038 / R .039
MWT as above 9� from muzzle

12g 1905 Remington 1894 A Damascus
End of chamber L . / R .
End forcing cone L .112 / R .107
9� from breech L .060 / R .058
9� from muzzle L .037 / R .035
MWT L .037 9" to 5� from muzzle / .035 9� to 3� from muzzle

very useful data. sounds like old ed's formula of .030 and .090 is a good guideline for what is safe and what is not.

Mark Beasland may chime in if he's not hunting or working, but I recall him telling me about wall thickness at the end of the chamber on the Verney-Carron 20ga he had made a couple years ago. I was surprised it was so thin. But that's modern metallurgy, which likely makes some difference.

Just as an outsider looking in don�t you people think it is about time that the United States Government started and supported a firearms proofing system. It seems to me that such a large and advanced country who can spend billions of dollars on this that and the other project, but cannot find the time money or inclination to protect its citizens from dangerous firearms, be they home produced imported or just adjusted by the home user. We read about the United States having an extremely powerful gun lobby well what are you waiting for it won�t happen unless you ask!!!!!!!
Or is my suggestion to impossible for you shooting folks to insist on having, though I would sincerely like to know why in reality it cannot be done or is it just politics.

Will I have to keep my head down to miss the flak?

This subject has been brought up here on this forum in the past & ran into many "Heated" pages. The Vast Majority of "US" were violently opposed to the idea. The big problem here is unless any gun sold was proofed to SAAMI specs for the shells the Average consumer is going to buy the proof law would be virtually meaningless. Personally I am quite content to load for my 100 yr old guns to a lower pressure & don't want them subjected to a proof to handle SAAMI spec shells. The US companies have been doing quite well for many, many years handling their own proofing without any Government Interference & have built some of the sturdiest guns available anywhere. In short it is a subject best Dropped.
Note well, you never see any warnings concerning firing British or other European shells in US guns, its always the other way around & all their guns have been subject to proof laws since the Dark Ages.

How could we be so backward not to think of that?!

April 20 1895 Sporting Life
http://www.la84foundation.org/SportsLibrary/SportingLife/1895/VOL_25_NO_04/SL2504014.pdf

July 27 1895
Call for Government Proof House for Nitro Powder Testing
http://www.la84foundation.org/SportsLibrary/SportingLife/1895/VOL_25_NO_18/SL2518019.pdf

Your colonial inferiors unfortunately chose to turn gun safety assurance over to the personal injury lawyers

I would like to ask would it be possible for you colonists (though the word colonists is sort of anathema here because I was certainly under the impression that your colony left the room, after one of our king Georges governments committed one all mighty political faux pas over that product we British stole from those kind Chinese people while we were dealing drugs to them. But that is now water under the bridge I hope!) Would you indulge me a little more.
On this side of the pond �Tort� which is a law appertaining to civil wrongs and the catch all meaning of it is �you cannot plead ignorance of the law�. So if you adjust tamper or in any way fundamentally alter a firearm the law compels you to have it retested by the official body which is in our case �the proof house� failure to do this puts you in breach of our criminal law and Tort if you sold it on after you made the adjustments. Therefore you have no excuses when it comes to whatever action is taken against you be it criminal civil or both because you cannot plea ignorance of the Law.
Now if there are no Statutes appertaining to �Gun Proofing� on your side of the pond and a person tampers with a gun to such an extent that it is positively dangerous how can you take legal action against such a person because you would now have to prove that he intentionally made the firearm unsafe. To counteract your argument he could say that I did not know his actions would cause such a reaction because he does not have many engineering skills. This is a lawyer�s version of heaven or as close to one on earth as it is possible to be or is this subject of Her Majesty QE2 perceiving it totally wrong.

Drew,
What is the curved sliver of metal to the right of your clock?

Bill made a .038" thick curved segment which I affix to the table with a C clamp and use to zero the gauge. Works great.

Damascus: I'll let someone within the profession explain personal injury law in the U.S. However, as has been expressed on previous threads regarding a U.S. proof house, most of us have a profound lack of confidence that our moral and intellectual superiors in Washington would not confuse 'safe' with 'it's black and plastic so it's evil' and then decline to prove, and effectively ban, those guns that are not 'good'.

Drew,
Thank you.
Just to throw a little common sense into the ring, yes we can have guns that safely pass the 'proof' procedure with barrel walls as thin as 0.015", but are you comfortable weilding a gun with walls that thin? Are you comfortable buying a gun with walls that thin to just stand and admire it or just occasionally shoot it? I like guns that are sound and shootable.

Here we're free to modify our guns as we see fit.

The word 'tamper' does not, thankfully, apply because it infers lack of authorization..


tam�per
verb
interfere with (something) in order to cause damage or make unauthorized alterations.


Some of us still have problems with 'authority'. It seems to be our basic nature here.

The accidents I've seen reported seem to not be directly related to irresponsible metal removal, but rather to operator error of some type.

You can hardly blame the gun or modifications thereto when a failure is caused by an obstruction or a loading error.

How do you 'proof' a shooter? Mistakes happen, just like in any endeavor. There are no examples of manmade things that have not failed catastrophically at one time or another. You get on the Titanic, you drive over a bridge, you put a shell in a gun.... you take your chances.

Engineers can't, experts aren't, and people are just people.

"... a totally unrecognizable genie has once again unbuttoned his pants and urinated on the pillar of science." - E.K. Gann

As shotgunjones pointed out, with very few exceptions we are free to modify our firearms as we wish. The exceptions typically have to do with alterations that might change a weapon's regulatory status as a so-called "assault rifle", machine gun, short-barreled rifle, sawed-off shotgun, etc. There is no law whatsoever (that I am aware of) that concerns itself with the safety of alterations or modifications of firearms.

Concerning torts (negligence), I am not sure a person who alters a firearm necessarily owes a duty of care to a person who later uses that firearm. It would depend on the situation and circumstances. In any case the plaintiff must have actually suffered a loss or injury and be able to prove that it was caused by a failure (of the defendant) to exercise reasonable care. Assumption of risk would likely be a defense, as second-hand firearms are typically sold caveat emptor. Common sales terms are "sold as-is" and "it is the responsibility of the buyer to have it checked by a competent gunsmith."

Thank you for the explanations gentlemen it now explains and makes clear a lot of things I always wondered about.

When it comes to second-hand firearms in particular, the rule in this country is pretty much "caveat emptor". If you buy from a gun dealer, some of them are very good about standing behind what they sell. And most would certainly be careful about not selling something that's likely to do damage to the new owner. But if you buy from a private party, the best practice is to take the gun to a knowledgeable gunsmith before you complete the transaction. If not, take it to one afterwards if you have any doubts concerning whether the gun is safe to shoot.

The other issue is that the seller here has no control over what sort of roman candles the purchaser may stuff in his newly acquired gun. After Guy Fawkes got a barrel of black powder there might have been some restrictions on reloading and components in England?

Drew firstly I must say that I am not the greatest exponent of reloading but I did give it a try some time back but I personally found myself thinking I would rather be doing anything but this reloading chore. I am sure that things have not changed here in Brit land since then so primers wads and Nitro propellants are all available for the home re-loader on production of that all-important Shotgun Certificate.
Now Black Powder is another animal entirely enter that process Brit bureaucracy does extremely well possibly because of Guy Fawkes and his barrel of Gunpowder who is fondly remembered by us Brits on the eve of November 5th. To purchase a pound or two of Gunpowder for sporting use so out come the hoops for the poor unfortunate would be user to jump through, so now you require a licence to purchase a licence to transport and an inspection of where the powder is to be stored which in my view made shooting Black powder one hoop jump to far. Though surprisingly British bureaucracy can be rather out of touch with reality, during months leading up to November the 5th retail outlets and supermarkets only need a retailer�s licence to store and sell Fireworks to the public which as we all know contain Gunpowder and their stock levels over the country must amount to tons of the stuff not to mention the amounts in fireworks kept in private homes over the November period.

Over Here we shoot fireworks on the 4th of July. Umm wonder why we do that & Ya'll don't.

Possibly for the same reason another country celebrates January 27th and you don�t!!!

For clarification, auction houses and dealers publish minimum wall thickness. So, the walls might measure over 30-40 thou in most areas but if there is a raised dent that has been cleaned out and a thin spot of 19 thou, the measurement given will be 19 thou.

Regarding the 7 thou proof pass. The gun was in an auction and was delivered asa n out of proof gun. It was cleaned up and re-proofed to make it legal to sell in tact.

Wall reduction tends to be either incremental as a result of lapping the entire bore to remove pits or spot located, where a dent has been removed or a bulge knocked back. The section behind the chokes going back for about 10 inches is the place where barrels are thinnest.