Although I almost choke on the words, don't forget to try some of the inline-type sabots with lighter bullets. Finding .50 cal. bullets that weigh 350gr. to 380gr. might be impossible or quite difficult, but the use of .50 cal sabots and .458 cast bulelts opens the doors for any weight due to the proliferation of bullets of taht size.
: I know you're having the .50/90 made to shoot .50 cal bullets, but for deer or other small game, a .458 bullet in a sabot might give you the light load regulation you want.
; I think there's a 330gr. .50 bullet Lyman mould still available, then the pointy 450gr. Lyman/Lee and 450gr. RCBS flat nose, 520gr. Lyman FN and 550gr. RCBS FN (these weights are with my alloys)
; One other suggestion - I shoot either straight WW metal in my .50's, or 1/2 WW and 1/2 pure lead. With a BP lube like Lyman Black Powder Gold, SPG or OxYoke stick, I've driven 450gr. bullets from the Lyman mould to 2,000fps without leading and 1-1/2" accuracy at 100 meters (iron sights). The WW bullets will expand or rivet somewhat, but the 1/2 and 1/2 ones are super expanders on heavy game.