doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Model 12 burst

Keith. Thank you for contributing that informative post. Really.

I asked the metallurgical engineer at METL how much barrels thinned as they bulged. He looked at me like "the formula is too deep for you" and said "not much".

You're welcome Drew. I'm going to say that it would be darn near impossible for even a PhD Metallurgical Engineer to provide a formula to accurately quantify the amount of thinning that would occur in these scenarios. There are just too many variables that wouldn't easily be repeatable or reproducible. A big factor is simply knowing what the actual pressure curve in that region was when it blew. I think we can safely say... "too much!"

If we could know the near instantaneous pressures involved, a guy with those credentials and that sort of training might be able to give us a good ballpark estimate. But a different barrel sample, even one made from the same heat of steel, that had some small internal flaw or inclusion, could throw his calculations way off one way or the other in a real world test. That wouldn't necessarily make him wrong. It would demonstrate that we are working with a material that is not 100% homogeneous and free from impurities. We also have engineering tables for accepted loads for framing lumber, for example. A hidden knot or bark inclusion can throw off the safe ultimate load there too. When the engineer at METL told you "not much", he gave you good information, because the difference between the thinnest point, and the dimensions just before and after was only .006"

Another steel that had different qualities of composition analysis, tensile strength, actual Rockwell hardness due to work hardening during rolling, etc., etc., may have stretched a bit more or a bit less before blowing out. But I think it's safe to say this barrel was doomed by the obstruction, and a shooter who didn't know about it. Like Lloyd, I too am interested in hearing what the actual analysis of this barrel is.

Interesting, while I can't say I've ever measured it myself, there's often comment about repairable bulges being appreciably thinner. Since it may not by practical to direct pressure spikes, I've wondered why all bulges aren't actually ring bulges that showed in a less than circumferential manner due to an area that was struck thinner or had some other weakness. I believe the proof process has shown that a barrel can be bulged or blown without obstruction.

Just how practical is it to repair a bulge? I don't want to seem disparaging here but...wouldn't it mostly be to fob a gun off on some unsuspecting buyer? Even if you do fix it and then seldom use it (because you know what might happen), doesn't that effectively guarantee that someday, somebody else will end up with it?

It is a problem that my brother is currently wrestling with.

Lloyd - it depends a great deal on the location of the bulge. A small bulge toward the muzzle where the pressure is low is not hard to repair. I don't think any barrel specialist would attempt a bulge repair near the chamber or proximal 1/3 of the barrel.

OTOH
Experts on Guns and Shooting, George Teasdale Teasdale-Buckell, 1900
http://books.google.com/books?id=4xRmHkr7Lp8C&pg=PA373&dq
On the subject of steel v. Damascus, Mr Stephen Grant is very clear, and much prefers Damascus for hard working guns. He related an anecdote of one of his patrons, whose keeper stupidly put a 12-bore cartridge into his master�s gun without knowing that he had previously inserted a 20-case, which had stuffed up the barrel. Fortunately, no burst occurred, but a big bulge, which, however, Mr Grant hammered down, and the gun is now as good as ever.


Craig makes a good point.
Thinned barrels usually just split, where they are most thin, which is usually the lateral wall - .018" here after "inexpert" honing to .739"



Obstructional bursts can be asymmetric if the wall is supported somewhat by adjacent metal, but the terminal burst will again be where the wall is most thin



A Parker 10g with what was very likely a wad in the forcing cone, with an asymmetric bulge, and lateral terminal burst. The inferior wall of the chamber was of course supported by the hook assembly

% elongation, a measure of ductility, is involved also but I'll leave it up to someone else to explain.

In the pattern welded tensile testing the average % elongation was 9.5%; with a high of 17% (Parker 6 iron) and low of 5% (Damascus Twist)

Fluid steels (yield psi - ultimate psi - % elongation):
1898 Armor - 57500 - 101000 - 12
(Non-standard AISI 1045 carbon steel with high phosphorus and sulfur.)
Crescent/LLH - 55000 - 104000 - 27
(Non-standard AISI 1040 Steel with high phosphorus)
Pieper Cockerill - 54,500 - 99,500 - 21
(Non-standard AISI 1030 carbon steel with high manganese & phosphorus.)
Sterlingworth - 86,000 - 103,000 - 15
(1040)
Krupp Essen - 89000 - 113000 - 16.5
(Dennis Potter gave me the Krupp segment but unfortunately the gun source and date of manufacture is not known so I did not composition analyze)
Remington 3200 - 113000 - 121000 - 13

Decarbonized Steel
Crescent - 41000 - 66,000 - 26
Meriden �Armory� - 48000 - 71500 - 29

If someone looks at stress/strain charts, as steel starts to go plastic, its strength exceeds its generic rating. It seems that thinning from bulging may be negligible. keith had mentioned earlier that cold working, one example maybe being bringing a bulge back down, likely increases hardness. If a good smith assesses the bulge and knows how to address it, why not? To me the most worrying desription of a classic gun barrel is 'mirror bores'.

Dr. Drew: Just looking at those pictures makes me queasy! I have read about the curious idea of repaired damascus tubes being preferred over steel (there seems to be a number of curious ideas about damascus, both good and bad). As much as I have enjoyed looking at and even handling some of Mr. Stephen Grant's guns, I can't imagine that any material that has exceeded it's elastic limit is "as good as new".

Wow Keith...I hadn't thought of that one. Does "cold-working" actually increase hardness? Is that even feasible?

This is another split discussed here some time ago, with wall thickness numbers. .023 -.027 stretched and thinned to .019"
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=13496&page=1

I usually pass on working on bulges simply do to the trade off involved. Rarely is a bulge completely removed without removing the ribs. The amount of work involved and costs associated with it rarely justifies the results in my mind. Safe enough depending upon location, wall thickness and extent of the bulge but just not usually a great option, YMMV.

Steve