I have some of the M4's and the AK74's. The only one I tried out was one of the AK74's (ASK204P.) The 8.4v 1400 MAh battery included with it won't pull the motor. So I also tried the following:
9.6v - Worked fine
7.4v 15C Lipo - worked but was weak
7.4v 20C Lipo - worked fine (just like the 9.6v)
I really like the way they sound, but since I'm in the US, it came with a huge, all orange flash hider! The stock locks nice and tight, but the upper receiver, where the battery is, has a little play when closed. The stock hicap that comes with the gun locked in tight, but I don't have any other AK mags to test with.