I have a 420 also, haven't shot it yet. Was suggested to me by a few guys to go with a .503" bullet(sabotloader mentioned if I recall that his 320 and his green mtn. barrel load the same and that measures .503" so hefigured good bet). Just ordered some 460gr bullets from Dan(bullshop)to try. Its also recommended to use wad between bullet and powder.

I'd sayideally one would want a bullet exactly bore size.