needs to be rock and roll, with a cyclic rate reducer, ala the Norrel design, to be really interesting. Folding stick would be a good idea, too. Along with a .22 unit that doesn't utilize that huge projection of a 30 rd .223 mag. I've cut the 30 rd Ciener mag to 20 rds, and it worked fine. Slow down the rate, from 30 shots per second to 15 per second, and you can fire 3 bursts out of a 20 rd mag, with excellent control/effect. 7 rds of 60 gr Aquila subsonic ammo, 120 ft lbs each, all hitting your chest at 15 yds, all in 1/2 second, is going to get your attention!
Without notifying everyone within 1/2 mile of you that you fired a shot. Even more so for a braced firing position (around cover) ambush, 10 yds and head hits. Sort of beats having to take the time to sneak up and cut a sentry's throat. All that thrashing around ain't exactly quiet, and the blood being all over him, the ground, and you is sort of problematic, also.