doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Shell Pressure through Inserts

How is a given shotshell's load pressure effected when it's fired though a sub-gauge insert? Also, are the different size shells effected differently when shot through different inserts? For example is pressure loss, gain or no difference the same using 12-16, 12-20, and 12-28 inserts?

Thanks

"We have commissioned independent pressure/velocity testing with Winchester AA and Remington STS ammunition in 12 to 20, 12 to 28, and 16 to 28-gauge adapters.
Pressure testing was done at the forcing cone area. Because different test guns were used, the 12g gun was measured at 3" and the 16g at 3 5/16" from the breech face. There are no industry standards for pressure at that point.
Here is a sample of the data:
16 to 28 Winchester AA -- 5900 psi and 1252 fps
16 to 28-g Remington STS -- 5100 psi and 1215 fps
Nominal velocity for both brands is 1200 fps.
SAAMI pressure standard for 16g at one inch from the breech face is 11,500 psi."
You might contact the GaugeMate folks for the other infro, and please let us know the results.
http://www.gauge-mate.com/index.htm

Seems most likely to me that the chamber pressure produced by the load would be the same as always. However, the added wall of the chamber insert would make the barrel stronger at that point, assuming there was a close fit between insert and chamber.

I use a Gage-Mate 16ga to 28 ga adapter in an old German single shot 16ga, it's very soft to shoot and I am happier using that greatly strenthens the chamber walls. I have a pair of Lil Skeeters I want to use in an old English hammer damascus SXS, but I have to turn them on a lathe to shorten/chamfer them so I can close the gun, they are a tad long. Anyone here shorten Lil Skeeters to fit a 2 1/2" 12 ga chamber?
Mike

The conservative (most safe) assumption would be to expect that there is no loss in pressure, measured at the powder chamber in the shell, for a given shell.

However, there are several other considerations on how that pressure is affects the barrel of a gun when the shell is a smaller gauge than the barrel. First, like Jim said, the Gage-mate will contain some pressure. Even if a little loose in the chamber, it has elasticity and will stretch to come to bear against the inside of the barrel chamber, while containing at least part of the pressure. Then, the barrel takes on the pressure containment. However, since the shell is of smaller diameter, the area the peak pressure is exerted against on the outside of ... say a .410 shell, might be ...say 1.5 in/sq. over a 1 in length from breachface forward (I didn't calculate it, just swag'in for discussion purposes). Then slip the 12/410 Gage-mate over it and the area on the outside of the Gage-mate that is from the breachface to 1 inch forward, now becomes something like 2.25 sq/in or a 50% increase in area. This is the area (all just swag's, it's probably a bit more) that will bear the pressure against the chamber of the 12g barrel. Since there is a finite amount of force (combustion of the .410 shell), distributing it over a larger area will result in a lower stress, even if the Gage-mate didn't contain some pressure (which it does).

Chuck;
Your figures are very close. I took nominal dia of chamber @ 1" from breech & calculated as if a straight tube of that dia & got 1.49 In² for the .410 & 2.54 In² for the 12ga. This amounts to an increase of 70% in area.

I have been considering taking a pair of 12 to 20 gaugemates and turning them down to fit a 2" chambered 12 gauge. That would enable me to shoot the gun with walllyworld shells. Way cheaper than two inchers for clay targets. Any thoughts?
Nial

Nial: you're likely to have a terrible time getting the spent shells out of the tube. Even with AAs, after just a few shots, I had to use a 'puncher outer' using the standard 12 to 20g tubes.

Before I did what you are considering, I'd need to know what pressures the gun is safe to shoot, what pressures the shells you plan on shooting are, and then I'd consider making the gauge reducers from a high tensile steel. I'm not all that familiar with what the common commercial Gage-mates are made from. If from aluminum, I'd sure give some thought to make some from steel. I'd also run the numbers to understand the stress exerted on the barrels from the 20g shell without consideration to the strength of the Gage-mate. Any added strength from the Gage-mates I'd just consider as additional safety margin.

The same amount of force is going to be spread over a larger chamber wall area, therefor lower pounds per square inch. Pressure will be the same on the inside of the adaptor, but lower on the chamber wall.