doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Minimum wall thickness

I have always wondered why the minimum wall thickness is generally the thinnest about 8-10" from the muzzle? This is where one generally places the left hand and the most likely body part to be damaged if a defective barrel.Any helpful comments?

Unless you have the arms of a professional basketball player I can't imagine your left hand would be 8-10 inches from the muzzle unless your shooting a very short barreled shotgun.

Chamber pressure peaks closer to the breech and continues to drop as the shot charge travels further down the barrels. By the time its down towards the ends of the barrels (assuming 25 to 30 inch barrels), pressure has dropped a ton. Accordingly, the further you get away from the breech the thinner the barrel wall thickness can be. Barrel wall thickness will increase when one gets into the choke area. For Foxes and Parkers (gun's I know a bit more about) choke length is about 4-5 inches long.

In summary barrel thickness up near the breech area in front of the chambers are very thick. Normally at least .100. By the time you get to the middle of the barrels, barrel wall thickness should hopefully be in the .035 range. Most people would like minimum barrel wall thickness no less then .035. That said, many well struck barrels left their factories with less then that. I've seen some original Parkers that had untouched bores but minimum barrel wall thickness was in the .022 range. That was indeed down at the far end of the barrels, just before the barrels started to gain wall thickness due to choking.

PS. Above is my 2 cents and I freely except opinions very and many many more folks have more data/facts then I do.

Actually, if you're right-handed your left hand will be more like 8-10" from the breech, not the muzzle (which is the very end of the barrel). This is where one's concern should be regarding the wall thickness, and that measurement (minimum wall thickness)is what is given by conscientious vendors for a shotgun they're selling.

Many people have their own standards as to what they consider safe, but .025 seems to be the general consensus as to what is acceptable for the minimum wall thickness in this area.

Now stand back so as not to get burned from all the flames that will come my way for spreading this very general theory.

Smokeless powder pressures, from Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, 1897



The English standard is 9" from the breech, which seems reasonable based on the pressure curve.

Thank you, Drew, for contributing this informative chart. I can't recall seeing it before, but I'm sure that's a fault of my memory as you seem to always be posting great "arcane" information regarding the guns we hold so dear and I'm sure this isn't the first time you've posted this one.

Again, thanks.

This is ground that has been covered many times here.




What the DuPont booklet goes on to say:


Pete

Thanks, guys for the information.

Tut, I am 6'5" tall and yes, my left hand is normally in front of the fore-end wood so I am concerned about my leading hand.

Pete, your graph is very informative and I take that to mean that the pressure declines very low after the first 12 inches from the breech end. Still, if the pressure is so low, why is .025 picked out as the appropriate number? Is that what proof houses use? I have heard that .020 is the minimum at 10" from the muzzle. Does it matter is the tubes are chrome lined or not? or if the gun is one made in the 1920's vs 1990's (assume steel in later guns is better).

Know this can be an endless discussion and I don't want it to drag on. Just looking for information.

Thanks, guys for the information.

Tut, I am 6'5" tall and yes, my left hand is normally in front of the fore-end wood so I am concerned about my leading hand.

Pete, your graph is very informative and I take that to mean that the pressure declines very low after the first 12 inches from the breech end. Still, if the pressure is so low, why is .025 picked out as the appropriate number? Is that what proof houses use? I have heard that .020 is the minimum at 10" from the muzzle. Does it matter is the tubes are chrome lined or not? or if the gun is one made in the 1920's vs 1990's (assume steel in later guns is better).

Know this can be an endless discussion and I don't want it to drag on. Just looking for information.

Actually, one of the problems when the walls nearer the muzzle get very thin is that the barrels are so easily dented. I believe it was Greener who experimented with very thin barrels - fired safely down to a thickness that could be cut with a knife.
I have a vintage 12 that passed nitro proof, but is just under .020 I believe.

There is no proofhouse "minimum" for wall thickness. The British gun trade recommends a minimum, but a lot of guns have passed proof with MBWT less than .020.

As the above graph illustrates, peak pressure always occurs within the chamber. That's true whether we're talking black or smokeless powder. And it always declines quite rapidly afterwards. Sherman Bell's more recent experiments with a variety of test loads showed basically the same type graphs as the Dupont tests from, I believe, the 30's.