Mag. primers are for large cases and I don`t think that the 40 S&W is.
What I would reather see you do is to load them in your 38`s. 2.5 to 3 gn`s of Bulleyes with a 158 gn. semi wad cutter hard cast with a mag. primer is a very good plinking round, and they are cheap compaired to other loads.