Good idea. Open use of standard mags, plus a single hi-cap for the rest of us. The MP5s will just have to outplay the others! Support weapons should be fully loaded, and would then actually serve their real world purpose.
There are only a couple of guys in the G3/AK category anyway, and the M4 is only a 100 round difference.
Jeff Ward played OSG 3 with an MP5 with ONE hi-cap mag and fared very well. it can be done.