Thanks for the info gentlemen.

Daryl, what bullet weight were you using in the .50 3Ľ? That case is pretty close to the .500 BPEs. The BPE rounds used bullets from 340 to 440 gn in that caliber. The light bullets were quite spry, at almost 1950 fps, and I suspect a faster powder would be needed with these. Wright suggested loads with the heavier bullet need to use a less than full case with 3F and 2F with a full charge, which seems to agree with your experience.

The 450/400 3Ľ used bullets with a SD about that of the 325 gn .50, although the case is bigger by proportion.