The M4 has had issues.

1. unable to add a magazine to the weapon from the inventory interface. I have to drop the weapon, and then add the magazine to the weapon on the ground.
2. weapon has no effect on NPC; no damage produced, nor are they reacting to the sound of the rifle firing. I have not fired upon player controlled characters, but can assume the same.

I can hear the rifle firing, but nothing is happen in the environment, zombies are not reacting or taking damage. When I log out and get back in a server, the rounds that I fired are repopulated as if I never fired a round.