Although you don't want to, the whole job is actually much easier and less time-consuming if you disassemble the mags. Basically, if you want to clean out all the cosmoline and
you want it done in the next couple of days, you're pretty much going to have to disassemble the mags.
Quickest and easiest is to break them down and run 'em through the dishwasher. No kidding. It works great, leaves no rust and doesn't seem to bother the dishwasher. I've run multiple milsurps and parts kits through the dishwasher without a problem. For 8 mags, I wouldn't even hesitate. For 2 dozen mags, you might need to consult wife or whomever is Household Six.
If you're pretty set on not
disassembling the mags, then heat and agitation are your friends. Do this outside.
If you can safely pre-warm the solvent (diesel, mineral spirits, kerosene all seem to work pretty much the same), do so. A GI .50 cal ammo can is a nice size to do this job.
Use a heat gun, oven or wife's hair dryer (not a torch) to heat up the mags and get that cosmoline melted, then dunk them in the solvent and swish 'em around energetically. After the 3rd mag or so, the solvent will be pretty warm, even without pre-heating. You'll want to cycle the follower up and down multiple times to try and get get the innards clean.
After that, stack up to 10 mags in a GI .30 cal ammo can full of Ed's Red* and leave 'em for a week or five. Every day or so, swish 'em around and push the followers down a few times. Be prepared for a long soak; some of that cosmoline is stubborn. Even at that, in order to get it all out you'll need to brush some of the corners, which goes back to it being easier to disassemble.
*Ed's Red is a home-brew gun cleaning solution that is dirt cheap and works great. Basically, it's equal parts automatic transmission fluid, kerosene, mineral spirits and acetone. Formal recipe here:
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