I just cleaned up my mags from Sportsman's Guide this weekend. I picked up some cleaner called Greased Lightning at Lowe's. It did a pretty good job of dissolving the cosmoline, except for the really heavy, dried deposits. I just let the mags soak in a bucket for a while, then lightly scrubbed them with a nylon brush. You definitely want to wear rubber gloves and glasses, though! (The same would be true with mineral spirits or any other solvent.) It was pretty low odor (I left the bucket sitting in the garage for two days -- no smell at all).
A couple of hours work and I had all 12 cleaned and ready to go!
Jim