doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com A question about barrel pressure

Larry, Miller,
I agree with your suppositions. However, the important factor is not pressure, when using a diameter reducing sleeve to facilitate shooting a smaller shell, it's the stress on the original gun barrel that we're really concerned about. Given the variables in the materials used for that sleeve, only straingage testing would be reliable in determining what the reduction (and there will be a significant reduction) in stress is due to a smaller diameter shell fired in a reducing sleeve.

For example, a 10,000 psi 410 shell fired in a 12g reducing sleeve that did not provide any additional pressure containment itself (theoretically), would exert less stress on the gun barrel than a 10,000 psi 12g shell fired in that barrel.

A 10,000 psi 20g shell fired in 12g reducing sleeve that didn't provide any support will exert more stress on the gun barrel than the 410 example above, but less than a 10,000 psi 12 shell fired in that gun.

Chuck;
What you say here is of course entirely correct. I didn't really delve into that portion of it in much depth, merely stating that while the chamber pressure would remain essentially unchanged one would add another layer to the walls.
As I understand the question it was addressing primarily as to what the effect on pressure would be forward of the insert. Here there would be no additional support to the bbl walls, but I cannot help but think the pressure would be lowered. To check this, pressure would need to be recorded at some point in a regular gauge gun of the smaller shell & then recorded at the same distance in the larger bore for comparsion. To be absolutely accurate I believe this would need to be done with a proper Piezo-Electric pressure gun, set up to record at say the 6" or 9" point.

I am curious if anyone has a chart showing the average pressures for a given gauge, at different intervals along the barrel.

Phillip, I'm not sure anyone has ever published that information, other than Sherman Bell in his "finding out for myself" series in Double Gun Journal. He used strain gauges at different intervals along the barrel. However, I don't recall that he did any of that with smallbore guns.

What about shooting 28 ga. shells in older 28 ga. guns? Do any of us make low pressure 28 ga. reloads or do we just trust the 100 year old guns to hold it?
Thank you,

I think maybe Miller posted a picture from one of his references that showed LUP gauges lined up down the length of a barrel.

There was a chart of this nature showing the pressure curves over the first 10" of a 12ga bore done By DuPont back in the 20's I believe. It was made for Black Powder & 4 of Dupont's then current smokeless shotgun propellents. A copy of this chart with only 3 of the smokeless powders appeared in the "Rifleman" back in the '50's. I am not set up for picture posting but did a scan of the Rifleman chart & e-mailed it to someone here, forget for sure who, & it has been posted to this forum on several occasions. Perhaps whoever has it will see this & do so once more. Beyond that 10" point as the expansion area is constantly increasing there would be a slow but steady decline of the bbl pressure.

I would be willing to put a beer on the pressure values fairly closely following the barrel taper of a modern gas auto like the 391 or some such equivalent.

Chuck;
It is amazing how closely the bbl contours of a pre-1900 SxS bbl follow those pressure values. It is also well worth noting that as smokeless powders began to gain wide acceptance gun makers began beefing up the "Chamber Walls" while the rest of the bbl's length remained virtually unchanged. In spite of this so called "Experts" continued trying to scare the public by stating smokeless loads produced their "Maximum" pressures some 10--12 inches down the bbl or right where your left hand was a-holding'er. This chart in fact shows that with all loads giving as near as possible the same velocity to 1¼oz of shot, from the 2½-3" point or about the end of the chamber forward the Black Powder was actually giving the higher pressure, while the smokeless was giving higher pressure in the chamber.

Mr Brown, thanks for the informaton. By chance do you know which issue I might find the article written by Sherman Bell in his "finding out for myself" which would contain the chart?