What does that term mean insofar as bore diameter, wall thickness, patterning, etc. Just wonderin', is all. RWTF
I have always understood riveling to be a surface distortion. I am sure it is a sign of weakness in barrels but perhaps not immediate failure. Not sure if it invalidates UK proof. As a rule I do not purchase any gun which is described as having riveling. I do think about very thin walls when I hear the term. Should have no effect of pattering. And it might be important to know where it exist. Past my fingers is a lot less scary than where I put my fingers. I was interested in a LeFever mid grade 20 bore which had riveling. Measured the bores and they were .011-015 where the riveling was. Needless to say I did not buy that gun. I own few guns below .020 and refuse to even think about what would happen if I shot even reasonable loads in a gun with .011 barrels. Nothing good I am sure. Riveling may also be the result of very large shot, in thinner barrels, so measuring might be the best course.
From the Hallowell site's Firearms Dictionary:
"Rivelled - A shotgun barrel damaged by having a series of slight ring-bulges or wrinkles. Caused by heavy loads or the reflected shock from a minor obstruction in the bore. Best detected by viewing along the outside of the barrel at a very shallow angle against a point-source of light. Very difficult to repair satisfactorily."
I don't believe that anyone can yet say for certain what causes rivelling in a shotgun barrel. The possible explanations for it that are given in Shotgun Technicana by McIntosh and Trevellian are as good as any. But I have never seen any credible reference that detailed conditions present when rivelling actually occurred.
I have seen several examples of guns that had rivelled barrels, and own one with minor rivelling that has been fired safely without further damage. All exhibited what appeared to be a series of very minor ring bulges or ripples along the forward half of the barrels, where the wall thickness is minimal. Strangely, although all were doubles, none had any noticeable rib joint damage. Minor bore obstructions seem to cause a single localized bulge or burst, so it doesn't seem likely that the cause of rivelling is mud or snow in the bore. The idea of something causing a repeated gas hammer effect sounds plausible, but it doesn't look like something that anyone has been able to duplicate to prove that theory. To see that effect in a test barrel in a ballistics lab, I'd think that you would have to have many piezo-electric transducers along the tube to record a big and very localized pressure spike exactly where the rivelling bulges occurred. And you would have to reproduce the conditions that caused a gas hammer effect.
I'm personally intrigued by the idea that excessive oil in the bore could become trapped under the wad to cause a series of hydraulically induced bulges or ripples as the charge accelerates down the bore. Logic would dictate that the relatively soft plastic wad would deform before the barrel wall bulged, but we have to consider the effect of several thousand pounds of pressure behind it. It would seem to be easy to test that theory with some junk barrels like Sherman Bell et al, but there are a lot of variables to consider such as the amount and viscosity of the bore oil, the type of wad, and the barrel wall thickness.
My old 10 gauge hammer gun, which was really a hogged out 12, had some slight rivelling in the barrels. MWTs in the distal third of the barrels was the low 20's so wall thickness was seemingly adequate. What I suspect happened is the seller's grandfather had used heavy 10 gauge 2 7/8 load in the gun for years, probably decades. That also explains why the gun was significantly off face. That issue aside, the gun operated fine once the barrels were shimmed back into place.
MTW in the distal third-- lemme guess- Minimum Wall Thickness- in the last 1/3d of the barrel length- guessing 30" barrels, so from the muzzles 10 inches back towards the breech? Is that correct. You didnt state what make this older hammer gun is. Care to mention that? RWTF
It was a Greener. Yes, the last ten inches of the barrel to muzzle. It was very subtle and I suspect wasn't a significant case of rivelling, but there was definitely a bit of a wave in the right barrel when looking at it under the light.
Bear in mind, this was a gun that was originally proofed as a 12 i.e. sub .740 barrels and now measured .783, then was subjected to very hard use. It was also Greener's laminated steel, which did very well in the Birmingham proof trials. As I mentioned, I think the prior owner's grandfather used it as a heavy fowler without much regard for the damascus barrels. Apparently held up, "well enough."