The issue with using slugs from the Mossy, quite frankly, isn't the ammo or the gun. It's the shooter.
Slugs are powerful enough for just about anything you'll hunt in this country. Yes, they will work at 100 yards.
Seeing, though, as the basic Mossy has only a front bead sight.....how well can you shoot it at 100 yards?
Unless you are going to scope the gun, even fifty is a lot under field conditions.
if you are going to stay with the smooth bore, Brenneke K.O. slugs are very effective.
Pete