The gun was designed to shoot 2 9/16” shells with a service load of 1 oz. shot at 2 1/2 Dr. Eq. (1165 fps) with a pressure of Bulk Smokeless about 7000 psi; Dense 500-1000 psi higher. Maximum load was 1 oz. shot with 2 3/4 Dr. Eq. (1220 fps) at about 8500 psi with Bulk Smokeless; Dense very close to modern pressures.
Parker cut the chambers slightly short for use with roll crimp shells. What is the chamber length now?
Using 2 3/4" shells in a 2 1/2" chamber will raise the pressure 200 – 1200 psi
https://docs.google.com/document/d/1...vwLYc-kGA/edit
What load to use cannot be answered without knowing the wall thickness at the end of the chamber, 3”, 6”, 9”, 12" and 16".