Ahh ok... sorry I thought you meant the stock standard BIS M2.
Hmm... tricky one.
I think either way you're going to need a variable to count something, be it magazines or bullets
The variable will have to be local, so either way you're going to end up needing to run one script per unit too 'keep' the variable for that particular unit.
I'm not sure if the fired eventhandler would work... dunno if it counts single bullets on full auto firing mode properly (I may well be wrong about that btw
).
I'd still lean towards counting the ammo, and detecting when the ammo > counter variable.
I'm not sure how you'd put that into a script bundled with the addon though.
I could write that script for a mission easily, but that's not really any help to ya ;D