I noticed my MP5 would stop shooting for no reason. I had a 30 round mag in it and at about 15 rounds in I wanted to single shot a zed and it didn't shoot.
I kept hitting "R" and dumping cartridges and still didn't work.
I changed to my spare mag and it fired again. My MP5 had "worn" status but bullets were pristine.

This bug also affects 1st Person.

Can confirm. I'm having the same issue. I'm trying to assign it to "mouse button 4" as i do in every game and it only shows the mic icon for a split second. Setting it to caps works fine though.

It definitely occurs when 2 players are firing at each other in close proximity.