The 20 gauge sabot slugs are really the direction that rifle barreled shotguns are heading. For deer, really no need for .50 caliber bullets, that is what the 400 grain Winchester Platinum tips are out of my 12 gauge
H & R Ultra Slughunter. The 260 grainers (.45 caliber I think) out of the 20 gauge, same bullet otherwise, would probably be just the ticket for deer. Unless deer start wearing armor, don't see needing bullets greater than 300 grains to knock them down.