A box mag is basically a "fancy" Hicap mag. Hicap (high capacity) mags work by using spring pressure to power a feeding wheel that picks up bbs and feeds them into a tube. The pressure that the spring exerts is necessary so that when the nozzle retracts a BB is forced up into position to be pushed into the chamber by the nozzle. Without the spring pressure the BBs will just sit in the tube. So hicap guys "tickle" their winding wheel continuously to keep the spring pressure up.
The "auto" box type have a microphone in them and are activated by the sound of the mechbox. When it shoots...it's winding...always keeping the spring tension/mechanism working, thus always keeping pressure behind the bbs to feed. The microphone picks up sound, activates a little motor, that actuates the mechanism to pick up bbs and shove 'em up the feed tube.
"Manual"/push-button boxes have a button that you press that activates the motor that actuates the mech...etc...etc...
Some mag systems tap into the AEG's battery and when the circuit is complete to fire it actuates the mag's motor that actuates the mech...etc...etc.. They may tap in via a plug...or via connector terminals/plates built into the AEG.
Basically go with whatever you want. The sound activated ones work. "Better" ones have a more robust motor and mech. "Cheapo" ones have crappy electronics, tiny motors and fragile mechs (that leads to jams/hang ups). Little motors running off of 2 AA batteries might have a hard time keeping up with hispeed SAW setups...you'll end up with blank shots in prolonged bursts, and hopefully no mangled nozzles, etc...
The "big green boxes" certainly hold a lot of BBs...but they're (at least for me) really awkward to get my support hand around when needed...and they make a hell of a racket. The smaller "nut-sack" type hold 1500+BBs and tend to be a bit quieter. You can also open up the box mag and line the holding area with padding to dampen the noise. Last setup I tried...I used the felt stickers that you put under tables so they don't scratch up floors...comes in rectangle sheets that I just cut to size and stuck to the inside.
Hope that helps
Last edited by m102404; January 6th, 2011 at 15:33..