Bolt catches & no dry firing on an empty mag
First off...aside from the latest GBB (gas blow back) rifles...no "typical" AEGs have a bolt per-se.
Some have a flimsy fake bolt made of thin sheet metal. Imagine a business card imprinted with an image of a "bolt".
On a lot of "old"/"traditional" AEGs...this fake bolt did nothing except hide/cover the mechbox. On some, it simply slid back so you could access the hopup unit adjusting wheel. On others...they weren't even movable.
On some "fancy" bodies...and on some makes (i.e. Star M4)...the fake bolt (still the thin sheet of embossed tin) would be retracted as you pulled the charging handle back and it would catch on a latch and be held back when the charging handle sprung forward. The bolt release would then trip the catch when you pushed the release tab. The bolt cover would snap forward with a little clink/click. Oooooh...aaahhhhh. Just about that exciting.
On some new G&G Blowback units...the thin tin sheet fake bolt cover has been expanded and operates on the movement of a piston. This piston rides piggy back on the main mechbox cylinder and air pressure is used to operate it. As the piston extends and contracts...the fake bolt goes clickety-clack back and forth with each shot. Ooooh...aaaahhhh. Noise every shot...
On a really old "Guarder Auto Back" system (I think it was AK specific)....you would mod your mechbox, install a rod that moved back and forth with the piston/spring. This rod would pull the AK bolt (which is a fake bolt...but not a little tin sheet) as the piston compressed the spring...and it would shoot forward as the piston went forward. Lots of noise....lots of action...busted up pistons/parts...
DonP modded a Triggermaster moseft unit on an M4 with a little pressure switch that sensed when a mag was inserted...and the chip "counted" shots and would force a mag change at 30 rounds. Never really took off...but awesome work and good stuff. His stuff is on Unconventional Airsoft.
NONE of these systems (except Don's) stopped/stops the AEG from shooting if there's no more bbs left in the mag. They'll all dryfire.
A Systema PTW (that's actually working properly...
) will cut off when the mag is empty. Pulling the trigger with an empty mag will do nothing. Dropping the mag and pulling the trigger will result in nothing. Insert a fresh mag and tap the bolt release...and it'll shoot again. There's no bolt, not even a fake one, so there's not clickety-clack of anything but the piston.
So...if you want the best realism available at the moment...(i.e. big bolt racking back with each shot, recoil, functional charging handle and bolt release, no shot if there's no mag, bolt locks back when empty)...get a GBB M4. $1300-1800 for the rifle, $50-100/mag.
NOTE: Most of this bolt stuff relates to M4's...the bolt of an AK doesn't lock back. Haven't seen/heard much re MP5's.