This one happend a few times to me now, anyone else too? I can only speak for xbox 360 btw.

When I rip-off a mounted HMG and then get killed the HMG drops. If I pick up the one I've lost a few seconds before it shows me unlimited ammo but I can't shoot with it. Whenever I press the fire button my character throws it away.

If someone else drops an HMG and I pick it up everything is fine. If others pick up my dropped HMG it seems to work fine as well.