G&G M4 - 2300 Round AEG Hicap DRUM Magazine (Auto-electric wind G-08-170-1).  This is the largest drum magazine you can get for the M4 AEG.  Fits in all M4 and M16 AEGs made by nearly all companies.  Screw midcaps - get this beastmode mag and dominate the entire battlefield with a smug face.

Battery is optional (see drop down options; charger not included).  Use a 7.4v small lipo that fits inside, then you can charge through the JST balance connection on the outside. Any 7.4v lipo will work, like a 250mah or 1000 stick should be fine.  Drum has a switch if you want to change from electric-wind to manual wind.

Full polymer build, easy to wind, load, and use.  Very smooth function for the most picky airsoft player.