doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Wall thickness measurement

Thanks for all the info. My question came largely from thinking about Damascus's go / no-go bore gauge. I was thinking about how that would affect the decision about a guns safety.

What I'm understanding is that if a gun is well out of proof, there's reason to suspect the walls might be overly thin. A gun being in proof is no gaurantee of good wall thickness but is a better bet.

Here's what I found on my Reilly, using a Manson wall thickness gauge and a skeets bore gauge:

Right: 0.655" diameter and 0.068" wall thickness @ 9" from breech. MWT 0.048" @ 9" from muzzle

Left: 0.657" diameter and 0.070" wall thickness @ 9" from the breech. WT 0.048 @ 9" from the muzzle and MWT 0.044 @ 6" from muzzle.

From the flats, I see the gun was proofed at 17/1 bore, which is 0.655". So the gun is well within proof (suggesting it hasn't been excessively honed, etc) and has plenty of wall thickness throughout to be safe to shoot.

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Apropos of nothing and offered only as a funny anecdote, not to support a viewpoint . . . I was talking to an old engineer I used to shoot with about what I thought were some very thin barrels that he was shooting with. “Aren’t you worried?” I asked. He said he wasn’t and I persisted “why not?”

“For the same reason that the walls of my cartridges don’t burst, pressure takes the path of least resistance”.

So when one oz of shot ahead of rapidly expanding gases exits stage left through the least resistance I might worry

I'll point out the elephant in the room.

How is blowing through a barrel wall ever less resistance than going down an open bore?

The answer disproves the engineer's reply.

There are several bursting formula:

Barlow's Formula P = 2 x S x 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 preferred the Alger Burst Formula
Burst pressure = Ultimate tensile strength x 3(OD – ID) / OD + 2xID

Lame Formula
Burst pressure psi = Ultimate Tensile strength x (OD squared – ID squared) / OD squared + ID squared

Mechanical engineers (plural) with whom I have discussed barrel bursts explained that all 3 apply to a pressure vessel; a pipe capped at both ends.
An UNOBSTRUCTED shotgun barrel is very very briefly "obstructed" by the ejecta, which very quickly begins to move in response to the gas pressure, which then rapidly falls.



SO the bursting formula do not work well for non-obstructional shotgun barrel bursts

Anywhere along the length of the tube where the gas pressure exceeds the bursting pressure, the burst becomes the "path of least resistance."

That is amazing the way the graph shows how quickly pressure dropped

not to worry...that graph should make us all feel warm an fuzzy...

This is exactly why I have so much problem believing that an overloaded (double charged, wrong powder, etc) shell, will ever burst the barrel wall several inches down bore. The pressure on that curve is down to ~2000 psi at 7 inches from the breech. That's less pressure than my JD tractors' working hydraulic pressures, and they are contained in some very thin walled tubing and run for thousands of hours at those pressures, including pressure spikes.

Until proven wrong with empirical data I will always believe a down bore barrel burst is caused by an obstruction other than the payload itself.

I once asked Mike Orlen about pitting near the muzzle end of barrels being a potential burst area and his response: "a garden hose could handle the pressures near the barrel's end." Gil

Assuming Stan:
Adequate wall thickness
Adequate tensile strength
A tube free of defects

Thin walls do split. A Perazzi MX 2000 bored out to .012"