The "old" style mags are a no go. The feed angle is perfect, but the 358 neck jams down in the body of the mag. I'd open it up a bit but there is not much meat to begin with. I might give Ruger another call and see if they'll send me a 450 Bushmaster mag. I saw where someone was having good luck with it in a 308. Only downside is that it is not a flush fit.