i had loads of space when i had tested the M4 out and the mag vanished, so maybe it was just a bug on my end then

@Captain Draco:
I haven't searched for an already existing bug ID covering this but as far as i know there is a bug which causes an empty mag to disappear if you don't have enough space in your inventory in size and shape of the empty mag (two vertical) when reloading. the full and the empty mag do unfortunately do not simply get swapped.

well, as it stands my mags vanish when emptied, even though i have rounds i could reload it with, but being able to get them back into your inventory when you reload would be great, especially if it's a partially emptied mag so the rounds left aren't wasted

I disagree. Empty mags should not drop to the ground by default.

BUT
It would be fine however if you could assign keys to two different reload mechanisms.
Normal relaod - empty mag goes to inventory, everything the same as before.
Speed reload - empty mag drops to the ground, allowing the survivor to put in the next mag a bit faster. Comes with the danger of not finding your mags or not being able to get them back in case you have to retreat.