doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com How much barrel obstruction ..............

.........is necessary to damage or burst a shotgun barrel?

Most of us likely know, and believe, that a barrel obstruction, as relates to the possibility of barrel damage, are things such as a lodged sub-gauge shell in a barrel, a lodged wad from a previous (faulty) round, mud, snow ............ in short, anything that completely blocks passage of the next payload. However, I was reading some very interesting things about that this morning and stumbled across an article by Tom Roster, arguably one of the world's top shotgun ballisticians of today. He mentioned in an article in Shotgun Life, in 2014, something I was unaware of ........... that grease or even oil, in the amount of just a heavy coating, can cause extreme pressure spikes. I'm not talking about an amount large enough to obstruct the bore as we might normally consider it, but just a heavy coating in the bore.

This is an excerpt from his article aforementioned. Especially interesting to me was the last paragraph.


"As a gross rule of thumb, obstruction bursts generally take place well down-barrel significantly past the chamber and usually closer in proximity to the muzzle than the chamber itself. The many forms obstructions can take include, but are not limited to, are an incorrect smaller gauge shell accidentally loaded which slips forward of the chamber and wedges in the forcing cone, a blooper load which leaves a wad wedged down-barrel, a plug of snow or mud caused by the muzzle being accidentally pushed into snow cover or soft ground, or excess grease and oil left inside the bore proper which is not properly removed prior to live firing.

A classic example of the first obstruction burst cause is a 20-gauge shell accidentally loaded into a 12-gauge chamber. But this can also occur from a 28-gauge shell accidentally being loaded into a 20- or 16-gauge chamber. Because the 20 gauge, more than any other gauge, can be involved with these two examples of a wrong gauge shell being loaded, U.S. manufacturers wisely elected years ago to color-code all 20-gauge ammunition a basic yellow color.

The oil or grease mentioned above may come as a surprise to even experienced shotgunners. It is a little understood fact in the U.S. that one test European shotgun manufacturers use to fast-proof barrels if standard proofing methodologies and loads are not available, is to liberally oil the bore and then fire a standard service load in it. The well-oiled bore interior will cause the pressure to rise some 6,000 to 7,000 pounds per square inch (PSI) above the Maximum Average Pressure (MAP) for service loads. And grease inside the bore can cause the pressure levels produced by shotshell service loads to easily double."


Likely, not removing heavy bore coatings of oil or grease, before firing, have also contributed to the rivelling and bulges we so often see in barrels.

SRH

Good stuff Stan.

Bruce Bowen�s H&R
https://www.trapshooters.com/threads/gun-blowups-past-thread.51234/
http://www.trapshooters.com/threads/h-r-detonation.146598/

Greener's (mostly unsuccessful) attempts in 1886
http://books.google.com/books?id=inQCAAAAYAAJ&pg=PA92

Experts on Guns and Shooting, George Teasdale Teasdale-Buckell, 1900
https://books.google.com/books?id=4xRmHkr7Lp8C&pg=PA372
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.

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.

Bottom line is that an obstructional burst can not be predicted, and is dependent on (in the absence of a barrel defect) the combination of wall thickness at the point of obstruction, pressure at the point of obstruction, tensile strength of the steel, degree of obstruction, hoop stress, and likely some other stuff we don't fully understand.


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.

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

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.

Wallace H. Coxe, in "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" published by E.I. du Pont de Nemours & Co. in 1931 cites a study in which a fluid steel barrel was cut to 9� and capped, then a series of progressively increasing pressure loads fired. The barrel cap was blown off and barrel burst at 5,600 psi.

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





Modern 4140

While not a shotgun barrel, I have heard of several .22 rifle barrels having the proverbial "ring" on the inside which many attribute to too much oil in the bore before shooting.

Note that it has been shown that at the point most obstructional bursts occur (12/20 combo etc excluded) "IF" the pressure were raised uniformly for the length of the bore behind the obstruction the barrel would contain the pressure without bursting or bulging. When the charge hits the obstruction it is checked in its acceleration down the bore & a pressure spike occurs at that point, not over all the bore behind it.

The fact of the bulge occurring ahead of where the wads were in the cited instance was because the charge was "Checked", not stopped. Velocity was high enough that it moved that far before the pressure hit its peak.

It has also been well proven the obstruction does not have to be solid, it's the weight that matters.

As to the matter of Damascus vs steel, & I love Damascus, its the yield vs tensile strengths of the barrels that determine a bulge or burst. As a general rule steel has both higher yield & tensile strengths than Damascus. Thus any pressure which bursts a steel barrel would burst the Damascus as well. Normally the range in which the barrel is bulged, but not burst is wider & starts lower for the Damascus. This does not prove the Damascus superior to steel, but actually the opposite.

Echoing the oil friction/pressure relationship:

"Remember to push a dry patch through before you next use the gun: oil in the bore adds extra pressure when you fire the next shot."

https://www.vintageguns.co.uk/articles/practical/266-2/

FWIW, I also think the etching of the bores in a damascus refinishing attempt (not by me), contributed to the stuck wad I experienced here:

http://www.doublegunshop.com/forums/ubbt...5264#Post495264

I guess they call them smoothbores for a reason...

I have read that a condition called rivelling which is a series of annular bulge rings in a barrel is caused by having too much oil in the bore upon firing.
Apparently the wad & shot ride over the oil accumulation ahead of them instead of continuing to push it on out the bore.

O.M

And I thought it was just an urban legend started by some bloke with too much time on his hands....

Another buldger of barrels is a paper case that separates from the brass. Firing paper cases over three times is a general danger. Examine potential cases for a burn line both case length wise and around the attachment to the brass.

A friend bought a new Marlin .22 rifle in the mid-1970s and before firing cleaned the bore pushing out a 3" plug of heavy grease. That was a wake-up moment!

Terribly risky to stick your shotgun barrel slightly underwater and then fire a round to get the birds moving like some old duck hunting buddies used to do just before sunrise?

Wasn't there an episode of Mythbusters where they shot various guns underwater?

Plastic shotshell hulls will also separate from the brass (or, in some cases, the plastic will split and result in a tear around the circumference). Most of the time, if you're paying attention, you know this immediately with a break open gun, because it will eject (or you will remove, if it's not an ejector gun) only what remains of the hull. Maybe just the brass; maybe the brass with only a short section of the plastic. In any case, at that point you need to take a good look down the bore and make sure that the missing section of the hull isn't lodged in the barrel. Usually happens with hulls that have been reloaded many times . . . but not always. I've always thought about guys shooting autos or pumps, who may not realize that what was ejected was only part of the hull. Very possibly big trouble the next time they pull the trigger.