WE PDW, kicks pretty much the same as an M4, takes M4 mags and has a far smaller and lighter bolt, allows for better gas efficiency and faster cycling.
plus, they're snazzy as fuck and small & lightweight
I disregarded the PDW 'cos I remembered reading something about them failing really abysmally in some particular area... But I don't remember the specifics of it... I'd quite like an MP7... Too many choices D=
Worth mentioning the kwa lm4 too. Trades are ugly but nit heard much bad about it, more realistic than the we internally too assuming you're after that
plus, they're snazzy as fuck and small & lightweight