I did reload before this happened and had full ammo, never fired a shot.
For the longest time, I thought this was an unfinished misfire system, but I just found out this is a bug. I reload and all that, click, the hammer goes down nothing. I've had this while crouching, out of formation, in formation, skirmishing, etc. It has happened with all weapons (minus the pistols), bayonet on and off. Every situation you can think of, I've had it happen to me, or I have seen it happen to someone else.