Im not sure why you would think a 1-20 would limit your bullet selection unless you want to shoot sub 180gr bullets at game???? The lighter bullets should still shoot fine with target loads that still beat 10mm performance levels.

A 1-20 shoots the Barnes 195, 200gr XTP and 200gr SST just fine. Those are the main 40cal bullets people currently use even in smokeless 45cals. Plus the faster twist should help if you want to buy longer/heavier bullets. 400gr+ conicals will likely benefit also and there are plenty of 458 molds to make you own heavy weights.