I tried Retumbo today with the 250 grain Hornady BTHP bullet and got the following results.
I didn't bother to go below 95 grains as the velocity was just not there. Did not try above 97 grains as I could not fit any more powder into the case and still stay within magazine length.
95gn, Velocity 2868 fps, case head diameter .547" compressed load.
96gn, Velocity 2925 fps, case head diameter .5475" compressed load.
97gn, Velocity 2958 fps, case head diameter .548" compressed load.
Some observations are that it takes about 2 grains more of Retumbo to acheive the same velocity as H1000. Recoil seems heavier with Retumbo. Can get higher velocity with H1000.
Next I will test for accuracy with a couple of loads in the 2920 fps to 2950 fps range. ................Stay tuned