Powder coating done well creates a slick surface on the bullet that also will survive the trip up the barrel very well. There are no drawbacks to the process. On sizing, cast bullets should be 1 or 2 thousands over bore diameter and you can run the bullet after coating through a Lee Sizer die if you wish. Since this is a 458Win Mag thread, you will find most off the shelf molds are .458 or .459 sized so the Powder coating helps get the bullet up to the .460 / .461 that you really want anyway. Of course you order custom molds in the proper diameter as well. Fear not. Coat away.
