I've been told "rebated rims won't feed" but I've managed to "wildcat" my 650 to do .50AE without any problems using stock parts, including automatic casefeeding.

The only thing Dillon would need to do besides putting some existing parts in a box labeled ".50AE" is to make a 650-specific .50AE powder funnel the same length as the .44 Mag funnel; the stock one for the 550 is just barely long enough, but a longer one would make powder feeding a little smoother.