If you don't know - JAM still doesn't get it to real life - there isn't a single JAM US 5.56mm STANAG magazine that can load into an M4, M16, etc....
OFP makes them break it down into firing modes as well, so the 5.56mm mags are divided into single/burst, single/auto, then a seperate entry for AUG mags, G36s, and so on.
If you know this already sorry, but the phrasing of your question seemed to indicate you didn't?