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).
Last edited by Chuck H; 08/12/07 03:04 PM.
