doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrels bursting

Humpty Dumpty;
I think the primary cause of burst bbls is an obstructed bore. Following that I would be inclined to take your Dad's Friend's opinion as the next most common. Actual Pressure Bursts, ie the pressure of the shell was too high for an unflawed bbl, even including damascus bbls being fired with modern shells, are extremely "Rare".

[quote=... Or should I believe a friend of my Dad's, who's a high-pressure pipelines expert - he says 8 out os 10 bursts are caused by hidden defects in the metal, which are enhanced by accumulated stress, and while the burst will probably coincide with a rise of pressure over everyday level (e.g. a slug or goose load), this pressure rise is simply a trigger, not a cause, of the burst. [/quote]

I assume he's talking about his experience after examining bunches of pipeline failures. The pipe used in his biz is likely made using good quality control techniques, yet he is telling us things still happen. Often, these 'hidden defects' are very small inclusions of contaminants. Some that I've seen were less than .003". I've seen many airplane parts made of the best materials available, using the some of the best quality controls, still have failures due to contaminant inclusions combined with high stress and repeated loads (fatigue).

One very important point, these inclusions require high stress repetively applied to generate a crack and grow it from the inclusion. How fast the crack grows is related to the stress levels, metal alloy, condition (hardness), oper temp, etc.. A crack that grows with repetive loads, then fails, can often reveal itself in the fracture face as 'striations'. Each striation is indicative of individual load cycles. The striations are usually extremely small, requireing magnification (no, usually not just a magnifying glass) to see.

O.k., that's it. That's about as far as I can take this with my single Metallurgy 101 class from college.

p.s.
But, to your acquaintance's comment, yes failures often initiate at 'defects'. In good designs, it is common to assume a defect is in the part at a high stress area. In airplane structure, we assume the defect is usually in the .050-.060" range because this is high probability "detectable threshold" we use for our NDI methods. Think about that for a while. If we use .050" because it is the smallest defect we have high confidence we can detect it, a shotgun barrel would be assumed to have a hole thru the thin section! That says to me that .050 is not good enough detection for a shotgun barrel. Heck, how many barrels do you suppose even got a NDI, ever? O.K. I'm done...well almost.

Yes, most failures originate at flaws/defects/inclusions. But my point is; good designs have considered the defect as existing in the part and have designed in sufficient margin to still do the job. Is that the case with barrels? I dunno, I'm not in that biz.

Barrel bulging/bursting can be pretty much understood from hoop stress analysis. Barrel ID, wall thikness and pressure are the factors for this quite simple calculation. To fully understand what goes on with barrel stress, you have to also know the barrel wall thickness profile and the pressure vs distance curve. These two are not exactly simple. Then, you need to understand the barrel material's strength properties. And, lastly, you need to know the barrel material's fatigue properties.

Your generic shotgun barrel has sufficient safety factor to withstand proof pressure at a very high level of confidence (very, very few barrels bulge/burst in proof). Also, steel is a good material for fatigue resistance. Although I haven't done actual fatigue analysis on a shotgun barrel, I'd bet sporting money on near infinite fatigue life.

Obstruction ruled out, we are pretty much left with thin spots or inclusions/discontinuities that act as stress risers within the material. CH pointed out that the technology to find very small inclusions is not available/practicle for shotgun barrels. If the stress riser acts to increase the actual stress within the steel near the riser sufficiently, we can get into a fatigue situation. How close the stress comes to ultimate tensile strength for the barrel material determines the fatigue life. One failure senario would be a lot of cycles that had heavily fatigued a thin spot or an inclusion followed by a load that produced unusually high pressure.

Thanks for the insites. I will continue with my philosophy that there is no need to load near max. pressures on any gun, even the newer ones.

LL - a wise philly. Lower pressure equals lower stress equals lower metal fatigue.

Here's a more simple answer, and we all love simple answers, right?

They made, and make, a lot of guns. For any given group of guns, there will be a percentage that have undetected flaws in the steel which lead to a premature failure. I don't know what that percentage is, but it is very, very small.

The number of guns which burst barrels is similarly very, very small.

Some guns with flaws will never burst - their "flaw vs. pressure" ratio will never be exceeded. Others with lesser flaws will burst as their pressure those flaws see will be greater. I'm sure "frequency of use" also factors into the equation.

So all we are seeing is the normal, natural occurrence of failures attributed to the normal, natural occurrence of manufacturing flaws. The guys who dump heavy loads through a gun are just increasing the likeliness that their particular gun will join the 'burst' crowd.

Some years ago, one of the Italian makers, I believe then A&S, had some barrel failures with their modern guns. They went to a Vaccum Melted Process on their steel for barrels as they wanted the barrels to last several lifetimes.

So that does tell us that even in modern times, that defects do happen with barrels. Then you attribute the barrel failures to very thin walls, the problem can be multiplied.

I would bet that lots of reloaders use faster burning powders that generate near maximum, when they could go to a little slower powder and drop pressures considerably. But lots I have talked to, say it takes too much powder get the same velocity, thus too much cost. They are usually high volume shooters.

Faster powders can work OK. 16 gr 700-X, Rem compression hull and 209 primer with a CB4100-12-B (Lightning) will push a 7/8 oz load to about 1200 fps with about 6600 psi and 20.5 gr PB will do the same thing with 6000 psi. Go to one oz loads and 15.5 of 700-X will do 1150 fps at 7500 psi while 20 gr of PB will "gitterdone" at 6300 psi. 600 psi difference with 7/8 oz and 1200 psi difference in 1 oz loads. However, most "old gun" shooters will accept 7500 psi as a pasable pressure. I use 700-X for 7/8 oz and move to PB for heavier loads for old and new guns.

Leonard,
Yes, the vac-arc-remelt process was toyed with in the late 50's and eventually Latrobe came on-line with a vac-arc-remelt plant in the mid 60's. This process allows for producing much cleaner metals with less probability of inclusions and elements not disolved into solution. Most aircraft grade steels are made this way today. I believe, but could be mistaken, that round bar 4140 used in barrels is commonly produced with the benefit of this process in the U.S..

Still, inclusions do occur as we discovered when tungsten inclusions were found widespread in titanium aircraft bolts that were made from recycled Soviet submarine scrap, even after the vac-arc-remelts and slag removal (saw) of the ends of the ingots, common to Ti recycling. The tungsten inclusions were from hand-welding with TIG where the tungstens had been 'dipped' and broken off in the solidified weld.

So, there's always some probability of an inclusion in metals, even using the best available processes. That's why critical airplane structures "assume a flaw" and we calculate crack growth from there and insert multiple inspections before the crack gets to critical length.

With barrels, I suspect engineers today, assume a flaw and mitigate the possibility it will grow by lowering stresses (make it thicker). At least I hope they do.

Rocketman, I agree with you fully. The American way, seems to be the more the better. And old ways die hard, as seen in the aging trap shooters that want 1 1/8 loads at 1250, you know the story. The general hunting/shooting public knows nothing of any of this. Hopefully the companies that manufacture will wake up and realize that more is not always better. And, that autos will function perfetly on 8000-9000 psi.