They have taken the servers down to rectify the problem that after update that a lot of peoples items were gone from inventory.

I have no accidently lost my M4 so can't replicate this any more. However before I did lose it, it happened again whilst the M4 was in my equipment slot with 1 in the chamber and a full 30/30 Magazine. Upon logging back into the game and placing the M4 into my hand the magazine was showing 0 in the chamber and 14/30 in the magazine.

update: this seems to happen even if the M4 is not equipped upon logging out and the amount of bullets that vanish differs. It doesn't happen everytime i log out though.