Many, many years ago I crowded the original prototype to about 1250-1280 fps with 185 grain cast Linotype bullets and heavy loads of Winchester 630P, long discontinued. I could get the same with heavy loads of Unique, but the muzzle flash was extreme. Bullseye worked about as well. Some of today's powders will probably work as well, but you're heading into terra incognito as nobody is going to publish load data for them in today's world.

The .451 used cases that were essentially cut-down and neck reamed .308 brass. They had a good bit less capacity, which raised pressures, but they also had much thicker walls over the feed ramp.

You need to be alert to frame/slide battering and spring sacking, and whether or not your pistol has had any throating done. Some of the Combat Master's out there have had a lot of the lower case support cut away trying to solve feeding problems. A ruptured case with these is no joke. If I were determined to go that way again, I'd try to rig some kind of buffer into the system to preserve the frame and slide. The very early blued guns had slides cut apart and welded back together from full-length 1911 clones, as did the prototypes. Such treatment is not recommended for them at all. Recoil's nasty too at those levels.

Pat