SOme (but not all) off the .451 size bullets will fit your bore perfectly without sizing. Some are too loose though and may need knurling and sometimes knurling is not even enough. Hornday XTP do fit my bore perfectly without sizing, but the rest don't. And every bore is slightly different. If you think you're gonna stick with saboted 40 cals you don't need the sizer right away. But if you eventually want to push your gun out to further yardages you'll want the sizer to experiment with different 45 cals.