While most of the responses seem to want to reference ghost bullets, I will try to actually answer you on topic. I have only had 2 M4s become completely unusable. I have noticed that often when I load into a server that I can't reload with the Y button, it doesn't do anything. So I have to go into my inventory, press Y on the mag so that it drops into my inventory, then highlight another mag and press B. Seems to HAVE to be done each time you enter a server or else you will find yourself in a gunfight unable to reload outside of your inventory. Both times my gun became unusable it was after a server reset, or after having left a server for a minute or two and then logging back in. It was not something you could just replicate, unless you tried just continuously joining servers.