Low-pressure Factory Shells

[Researcher]

The OP stated his gun is of 1926 vintage. That is four years after the progressive burning smokeless powder, high velocity, 1 1/4-ounce loads --Western Super-X, Remington Heavy Duck Load soon to morph into Nitro Express, Peters High Velocity, etc. came on the market.

Early Super-X 12-ga box, Super Excellant.jpg

NITRO CLUB 12-gauge, Game Loads, Heavy Duck Load #5CH.jpg

HIGH VELOCITY 12-Gauge, 2 3-4 inch, #6Ch, Blue Wing Teal 02.jpg

And a couple of years after SAAMI specs came into the U.S. arms and ammunition industry.

The issue isn't that will the metal of a sound gun handle these loads, it will, but do you want to subject nearly 100-year-old wood and your shoulder to the recoil forces.