doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com A question for the engineers among us...

A few discussions on pitting recently have led me to ponder two questions, I'm too lazy to look up the equations to figure it out myself, I'm sure someone will know..

Assuming two identical single 12ga. barrels, same bore size and wall thickness. Barrel A has a 0.004" pit on the interior, barrel B has a 0.004" pit on the exterior.

To repair barrel A the I.D. is opened up 0.008, to repair barrel B the OD is reduced by 0.008. Which leaves a stronger barrel, less likely to give way under pressure?

I know that taking the material off the inside results in less material lost. OTOH the proof house will declare a barrel out of proof if the bore gets too big but don't specify any lower limits for wall thickness if metal is taken from the exterior and the bore left alone.

I specified single barrels since I realize that when making the same comparisons with SxS bbls. the area under the rib comes into play. My question boils down to this... would you rather have to strike pits off an exterior or hone them out of the interior (all else being equal, don't factor in rebluing, etc., just on issues of strength).

The second question is which is more likely to give way, an interior pit or an exterior one of the same depth? I would think it's better to have the constant surface on the interior so an exterior pit would be stronger. Right or wrong?

Thanks, Rob

That's my line of thinking.

I had one gunsmith/seller (Elephant and Castle) tell me he could measure the depth of interior barrel pits...I'm still scratching my head on that one.

I bet he's got different size roaches he lets run down the barrels.

Rob;
Not an engineer, "But" it is generally understood i believe that a smaller tube with same wall thickness is stronger than a larger one, so removing metal from the outside should not weaken it as much as removing from the inside. In the case of two tubes of nominal 12ga size a difference of only .008" would of course be mostly inconsequential.
As to the 2nd question, I think the important point there would be which would be most likely to "Grow" & get worse. This I highly suspect would be the internal one. as it would be exposed to firing residue & resist proper cleaning more than the external one.

Homeless I can measure the depth of most large pits also. I modified my barrel wall thickness gauge. Took out the round ball that use to press up against the inside of the barrel and replaced it with a pointed piece.

Rob,
As Miller has stated, a tube of a given wallthickness is able to sustain higher pressure if it is of smaller diameter. However, in practical terms, the .008" diameter changes are so small of an impact to a 12g tube that it is likely less than pressure variations shell to shell.

As for a flaw on the inside or outside of a tube, the outer surface sees higher stress than the inside. You decide whether additional damage from residues is likely to be of concern. On a blackpowder gun it would definitely be a concern. Smokeless stuff with non-corrosive primers? I'm not convinced since I have several guns that have had stable pits for years.

I would like to see a gun which has failed because of a single pit. Might be able to understand if you were talking about an area of deep pits causing the metal over them to be much thinner and weaker. It is all about location. A small deep pit in the chamber area has more metal over it than it wold just 9" down the bore. Pressure does drop the further down the bore you go but it should still be high at less than 10", I would think.

This would make a good study. Take a gun and bore a hole, with a ED-drill so the hole can be very small, like a pit, at about 9" from the chamber. Load a series of shells to increase pressure from 5,000 psi to 10,000 psi to see if and when it fails. A Crescent would be a good gun to use for this. Decent metal and fairly modern gun should be a good test model.

Thanks for the opinions.

KY John, I'm not talking about practical application here, I don't have a gun with a single pit. I'm inquiring about a theoretical barrel, trying to get some data from to extrapolate with. Talking straight physics here.

Also, in regards to the measurement of 0.008, it also was an arbitrary number but I'm surprised that Chuck & Miller think it would be inconsequential in a 12ga. when 0.010 over bore is when the English Proof Houses (keepers of the flame, so to speak) say it would need reproof.

As a practical matter, with the .004 pit on the exterior, you wouldn't remove material from all sides. You would just hopefully remove the pit by restriking that area and feather into the adjacent area. With the .004 pit on the interior, the only way to do it would be by honing and I agree that would take .008 material out of the entire bore.

weld an exterior pit.

Rob,
I didn't intend to convey that the .008" change to wallthickness was inconsequential, but rather that a reduction in outside diameter by .008 compared to an increase in inside diameter of .008 was likely inconsequential. That comparision would have both barrels of the same wallthickness but different diameters by .008". This diameter difference, IMO, would be somewhat inconsequential in practical application, even though theoretically the smaller tube with the same wall would be able to contain higher pressures.