Rockie, looks like you have a gun made between 1896-1904 by the proofmarks you listed, not reproofed. RST 2 1/2's would be fine. Pressure standard for the 2 1/2" .410 per SAAMI specs is 12,500 psi. Too hot for that old girl.
Larry, it has been my understanding that when using a chamber adapter such as Chambermates, Gauge Mates, etc. there is a significant reduction in pressure. I do not have any specific data, but I'd have to believe that a .410 loaded to SAAMI specs is yielding a whole lot less pressure when shooting that shell with a chamber adapter out of a 20-ga. gun. Am I wrong?
