Was wondering what the approx. operating pressures in the 500-577-600 type of double ammo.?

Welcome Bob96,
The original load data for the 500 is 570 gr bullet at 2150 fps 16.00tpsi. The 577 load was 750 gr bullet at 2050 fps at 14.0 tpsi. the 600 was 900gr bullet at1950 fps 14.0 tpsi. (Tons Per Square Inch) Yes this is a obsaleat English method of measuring pressure and is not convertible to any other system.
You don’t load a double rifle to max velocity and pressure like a bolt gun. You load to meet the original regulation load. This makes the rifle shoot to point of aim with both barrels.
Bill