On steel and damascus barrels, how would 18 and 20 thousands be?

Go to the auto parts or hardware store and pick up a feeler gauge. You can tell exactly how thin 18 and 20 thounsands is (damn thin!).
http://www.sears.com/shc/s/p_10153_12605...=33-135365945-2
Steve

It depends on WHERE the "thin" is. At muzzle... just cosmetic. At breech... BAD

Count your fingers now. After firing count them again. Is it worth it?

at 18 thousanths Id be afraid to grab the barrel much less shoot it....

How much money for the gun? I would not invest big bucks in a gun with thin barrels. Safe or not thin is not a selling point in my books. You get what you pay for if you know what you are doing and putting big money in to any gun with barrels that are .002-.005 of needing to be sleeved is not a good idea. Some would consider them already in need of sleeving. I prefeer .025 as a decent thickness and when a gun nears .020 my interest drops off quickly. But location is everything on these issues.

If near the muzzle .018-020 is not too thin but it is near the min. acceptable thickness. If within 20" of the breach face I would pass. Thin barrels, near the muzzle, are a greater denting risk than bursting risk. That is if you use common sense and use proper low pressure loads. If you shoot 10-12K psi shells then the barrels might rupture before the stock splits.

I also check if the barrels are uniformly thin from end to end or is it just one local area. If there is very little metal on most of the barrel from end to end I would consider the barrels to have been reamed to death. If it is just one area near the choke it might be fine to shoot. It was just cleaned up to remove a area of pitting. Bottom line is do you want a gun with barrels on their last leg? Because you will never be able to ream a single time more or even hone lightly. They are at their end of reaming.

I think I read where Holland & Holland are now saying anything under 21 thou. is too thin for shooting.

Don't fool yourself about thin walls being OK at the muzzle end of choked guns. Here's a set of Krupp steel barrels on a graded Fox that were struck to remove surface corrosion before rebluing. They seemed fine until about 100 shots were fired through each tube..... 1 ounce lower pressure lead loads on a sporting course. One pic is worth a 1000 words; that's a bulge, gents. Choke wall is .018" thick where shown by the arrow. Both tubes measure about the same in that area and both have bulges. Silvers

Frank, I know nothing about Foxes, but it seems to me that area for the choke is very short. Most guns start at around 5" from muzzle have a smaller diameter for the choke area and so have a heavier wall thickness in this area. If someone filed this area that small, then the wad is bulging this.
By the time the pressures reached this area they were significantly lower and in my opinion did not cause this.

Frank,
Can you provide any information on what the inside of the choke profile looks like in this gun? (where the taper starts, how much choke etc.)

Had a Greener once that did the same thing--long before I had a wall thickness gauge. It is since long gone.

Bores .724/.725", chokes ~ 4-1/2" long. Both as normal for early Fox 12 ga guns. Both chokes were opened from .039/.040" to .025" with a parallel section at the muzzles; choke work was done before this happened. Parallel sections were probably about 1" long which is about where the bulges became apparent but they may have started farther back from the muzzles. Walls were measured with a good Galazan vertical wall gauge. All the preceding from my records.

This did not result from stuck wads in one or both barrels.

Sorry, I can't supply more info because the ice is too thick on my neighbor's pond and the water is too cold to go diving for the barrel that's been there since I tossed it. RIP. Lesson learned on avoiding thin walls at the muzzles, on choked guns you really want to shoot more than a few shots per year..

That's scary as fuk!

I know of genuine original H&H barrels struck down to 0.019" to achieve weight balance as new.
But this gun was intended for use in the UK with normal UK nitro 2 1/2" loads. Nowadays, guns could be subjected to all sorts of pressures/loads in different countries: from high brass tooth-rattlers to gentle low pressure and no doubt H&H's recommendations reflect this.
I have successfully re-proofed Damascus down to 0.014" and, although I would not recommend this, it was safe to shoot afterwards.
When considering the location of thin parts of a barrel, it should be noted that although a thin patch can be anywhere, in a post 1880 English gun it is most likely to be found at around the half way mark.
Barrels made after the early hammergun era and beginning of choke era, were generally struck up with the 'Eiffel Tower' profile: ie. wide at the breech (to contain pressure), thinning toward the half-way mark (to save weight and aid 'balance') and then thickening (albeit rather less) towards the muzzle (to protect from dents).
A muzzle area as thin as 0.018" would be most unusual in my experience but I agree with JDW that pressure there is so light as to be unlikely to be the lone cause of that ring bulge.

Some new scary stuff here Jay
http://www.picturetrail.com/sfx/album/view/17546456

Don't confuse Max pressure with muzzle pressure. What the maximum chamber pressure is has little to do with remaining pressure at the muzzle. In fact if you push the same shot load to the same MV with two diffwrent loads the one having the lowest max pressure will almost certainly have the highest muzzle pressure.
"In My Mind" these bulges in the choke confirm that some component of the load, be it the shot charge itself or merely the wad, but some component is "Checked" just enough in its velocity to produce a pressure spike. This presents a situation just like an obstruction, though the load itself is the obstruction. This would occur irregardless of the wall thickness, though it doesn't become a problem until the bbl wall gets excesssivly thin.
I cannot say this is universely followed but I have checked a good number of guns on which the bbls were sort of "Swamped", ie the od was slightly larger starting a bit behind the chokes giving an increased thickness going into the chokes.
I would personally prefer to have that .020" thickmess at around to 18"-22" point than either near the muzzle or much closer to the breech unless the gun is a cylinder bore in which case it could extend all the way to the muzzle.

My theory is that it was mechanical constriction of the payload (wad-shot) that bulged Frank's barrel, not gas pressure.

Just a swag, but it would take about 3200 psi to yield a .700 bore/.018 wall 4130 normalized tube. I don't think that's likely. The tube is more likely than not thinner just behind the max constriction and therefore more likely to yield there from gas pressure, rather than at the maximum constriction/thickness area. Also, if the payload suddenly slowed at the choke, the payload itself is something like 3/4"-1" long and that would put the base of the wad behind the maximum constriction/thickness area where it would exert any spike in pressure (and therefore, bulge) from a theorized gas dynamic pulse.

As usual, Toby posted pretty much exactly the points that needed making, so I'll endorse them rather than post them again. Saved me a job!

Not necessarily. Even if we consider a leisurly MV of 1100 FPS & a total load length of 1.5" (probably both faster & shorter in actuality) the the load is moving at 13,200 inches per second. At this rate it would take .000114 secs for the total charge to pass a given point. Tests conducted with intentionally obstructed bores have definitely proved the bulge or burst usually occurs at a point farther along the bbl than the obstruction. I do not believe that gas hammer should automatically be ruled out here.

What are the specific physics that support a pressure pulse of approximately 50% of max chamber pressure in a localized area of the size seen here?

Chuck;
I never studied physics so I guess I can't specifically answer your question. We are all though I think familiar with obstruction bulges & bursts. The physics would be essentially the same with the choke serving as the obstruction. Sir Gerald Burrard devoted a great deal of space to this subject. A through perusal of it is most enlightening.

I was surfing thru some gas dynamics blogs and didnt run across anything applicable. The stuff I stumbled on didn't specifically identify localized pressure spikes. Nor did it discuss any fractional slowing of a piston in a cylinder. However, I did read about a shock wave in front of a piston in a cylinder and some stuff about amplification when the cylinder area reduces.

So how thin were the walls at the burst? 5/1000th or 50/1000ths?

If we are talking the last third of the muzzle end of the barrel 21/1000th is plenty for me. If we are talking a 1/2" from the muzzle end of the chamber it is not.

Does the story that goes with that picture give any theories of the failure?

An obstruction.

If it's an LC Smith, my theory stops right there.

There's some explanation in Doc Drew's link.

Thanks Craig, I missed that.

This is what Drew's caption to that picture said:

"These images are entertaining only after knowing that injuries sustained were not life threatening. Please be careful out there!
1918 12g L.C. Smith Field

Ruptured barrel with first shell of Bismuth Magnum Game Load 2 3/4" 1 3/8 oz. shot"

Mike: I'm sorry but I don't have any more information. As I recall no attempt was made to measure wall thickness.

Thanks Drew. I would be interested in sending my wall thickness gauge to the gun's owner and ask him to measure the wall thickness at the burst. Of course in that configuration a micorometer would work too!

If one's needed, I have a micrometer with a round anvil for measuring tubing wall thickness.
Steve

Sorry, I wasn't trying to draw inferences between the pictured incident and the OP's question. Just wanted to show the seriousness of a failure.
CHAZ

A few years ago I was talking to a friend who had recently bought a Birmy Boxlock. The first time he shot a box a shells through it a bulge appeared just ahead of the choke. He sent it off to his favored gunsmith and had the dent taken out. Next time he shot it the same bulge reappeared.

I can't recall exactly what the product was, but I recall a process where an irregular profile was machined on the o.d. of a tube and the i.d. was a straight hole. The part was then swaged into a die that made the o.d. straight. The i.d. that resulted had the irregular shape as though it was machined. I think it was a rocket motor case or something like that.

My thought was that this bulge in Frank's gun is the imprint of the inside profile where the payload swaged the barrel out.