doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrel ring bulge revisited

An obstruction is certainly a problem. Not long ago, I believe Toby B. commented on these forums about the various proof failures he had experienced over the years. Bulged barrels were mentioned specifically. Of any incident that created a bulge in a shotgun barrel, it's probably safe to conclude that an obstruction would be documented to be not presents, and the specification accuracy of the ammo should be beyond question.

If it can happen under the controlled conditions of proof testing, maybe over pressure alone can cause a shotgun barrel bulge out in the general shooting community.

Don, is it true that, given equal obstructions, one nearer the muzzle is more likely to cause a burst barrel, than one nearer the breech? My reason for asking is that it would seem that the more air that is trapped in the barrel, the more severe the air hammer will be.

A barrel that has been jabbed into the mud, then fired, will likely burst or at least have a severe split and/or bulge. However, years ago on here we had a poster who taught kids to shoot in some type of structured setting. He said he wanted to show what occurs when you drop a 20 ga. round down bore in a 12, then chamber and fire a 12 in it. He did so from a remote location, several times, with no bulges or burst barrel, and was incredulous. Most of us who read it were, too.

I realize all that is only anecdotal, but does it support my question in any way?

SRH

Barlow's bursting formula P=2 S t / D
P=Bursting pressure in psi.
S=Tensile strength of material in tube wall.
t=Wall thickness in inches.
D=Outside diameter in inches.

Burrard used the Alger Burst Formula
Burst pressure = Ultimate tensile strength x 3(OD � ID) / OD + 2xID

All the burst formulae refer to a pipe capped at both ends with a static pressure (a pressure cylinder). Shotgun barrels are not designed to be pressure vessels as one end is open and the pressure rises and falls quickly.

The Hoop Stress Formula doesn't reliably predict shotgun barrel failure either
https://www.engineersedge.com/material_science/hoop-stress.htm
Shotgun barrels are �thin wall cylinders�
stress = pr/t
p= pressure; r is the inside radius; t is the wall thickness

Barlow's (and the other formulae) DO work with a totally obstructed barrel. When the "critical confluence" of variables meet, the barrel bulges or bursts.

At least by the images I've accumulated, the point of the obstructional burst (not counting mud/snow in the muzzle) seems to be 6-12" from the breech





jOe's indestructible 4140



Thin barrel splits seem to be mostly 14-18"





And over-pressure loads blow out the breech

Obviously those barrels which Bulged in proof were not likely to have been obstructed. I "Believe" that normally proofing is none with a powder designed to stress the forward portion of the barrel to a greater extent than would be done with normal shotgun powders. My take would be the proof load exceeded the elastic limit of the steel at the point it bulged.

I would also expect that fully 98+ % of barrels bulged in use, either hunting or clays were caused by an obstruction. Also keep in mind that when an obstruction is hit the pressure in entire bore area behind the obstruction is not raised to the same level as At the Obstruction. You get a localized pressure spike near the obstruction, depending upon the severity of the obstruction. In some cases the bulge will actually occur a bit forward of the actual obstruction, showing the obstruction was moving before the pressure spike occurred, the gases simply caught up with it.

In the case of the Lefever I mentioned I feel the "Dent" was severe enough to cause an obstruction. I cannot think of any other reason the side opposite the crack would have been raised. Personally "IF" a barrel of mine gets a dent, I want it raised. I'd rather be safe than sorry.

At a local gunshop that I frequent, a very nice Browning o/u came (can't remember the ga.) in with both barrels bulged just in front of the hand guard. We all speculated about the cause and it eventually came out.
The fellow had been using sub guage chamber inserts that left a powder fouling build up/ridge at the point of the bulges. This fellow didn't clean the bore before he went back to shooting regular ga. shells and the barrels bulged. The general consensus was that he scrap the gun.

The posts and pics raise the issue of repairability. It is hard not to think of the ease of barrel replacement in the Baby Bretton (hand screwable ribless barrels). Especially when seeing how damage to one part can write off the whole barrel assembly in conventionally built guns. The design challenge would be to incorporate that kind of repairablity in something prettier cause the BB is undeniably ugly.

What Miller said

A study by the Royal Military College of Science, sponsored by the Birmingham Proof House and the British Association for Shooting and Conservation, showed that an obstruction by 2 fibre wads (total weight of 4 grams) was sufficient to bulge or burst a 12 gauge barrel shooting a 28 gram (slightly less than 1 ounce) load. Peak pressure occurred 22mm (.866�) past the leading edge of the obstruction.

And more from Burrard
If the burst was not caused by an obstruction it must have been the result of some excessive pressure or of some abnormal weakness in the barrel. A (excessive load) pressure burst can only occur in the immediate neighbourhood of the chamber; and so if the burst occurred ahead of the chamber cone an excessive pressure can be ruled out.

Another chamber burst



BUT an asymmetric ring bulge related to the addition thickness of the brazed barrel flats and support of the right barrel. The burst was likely a wad in the forcing cone



Federal 10g with 2 piece fiber base wad

While we're at it, properly designed and fabricated shotgun barrels, manufactured with the appropriate steel do NOT undergo low cycle fatigue; elastic deformation when the yield strength is exceeded.

This obviously is not a shotgun barrel but demonstrates the "waves in the sand" fatigue striations



150X SEM with fatigue striations transitioning to "fisheye" elastic deformation (stretching) then to cleavage



As said, I only play a metallurgist on DoubleGun and for additional light reading:
http://www.martin-moeser.de/Images/Fractography.html
https://www.efatigue.com/training/Chapter_3.pdf

Mr. Hause. As I read your posting I wanted to ask; all shotguns that fail proof in British proof houses would then fail at or very near the chamber, correct? Is there a way to know this for sure? Can it be safely said that any barrel burst further down the barrels must be due to an obstruction or structural defect ie the metal is defective in a significant way? I am really trying to get a handle on this idea that dents are minor bulges are major. Thank you.

In 2006 the British Rules of Proof were modified to apply increased pressure further down the tube
http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDF
Part IV, �The Proof Load�, Number 27, Part A
Calls for a load of 30% over mean service pressure at a point 17mm or 25mm (about 1�), and at a point 162mm (6.38�) from the breech face a load of 30% over mean service pressure (at that point as determined by the Proof House).
The target pressure at 6" and powder used is a Proof House secret

1929 Hunter Arms Proof Load Pressure Curve Chart
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107
12g 2 3/4� was proved with 6.5 Drams FFFg with 1.687 oz. shot = 14,200 psi at 1�; pressure at 6� was about 9,500 psi.
Pressures were measured by crushers (LUP - Lead Units Pressure) and modern piezoelectric transducer measurements would by 10 � 14% higher

A barrel that is dented 6" from the breech received significantly more force than one dented 6" from the muzzle; likewise a bulge 6" from the breech would be a major concern, 6" from the muzzle a minor concern. Make sense?
What the metal deformity looks like microscopically with either a dent or bulge requires a photomicrograph, with destruction of the barrel.

I would suggest "minor" or "major" is primarily a function of the location of the defect, and the wall thickness of the tube at the defect. And that can be measured without sacrificing the barrels.