I don't see it in the known issues list, so I'm reporting it: in several hours of playing Sandstorm with a friend I experienced this once and he did several times: there are rare situations where you pull the trigger and your gun won't shoot. We both have over 500 hours in INS2, I can 100% guarantuee that none of these situations where due to being out of ammo or having made an incomplete reload cycle that leaves the gun without a mag. I think (not 100% sure though) the one time it happened to me, I was coming fresh from the spawn and was messing around with the laser sight on/off button. When I died (because my friggin gun didn't fire) I think (again - not 100% sure) I cought a glimpse at the ammo display in the bottom right corner showing the magazine offset by one slot. Is there maybe a chance that something causes the gun to think it has neither a bullet in the chamber nor a magazine, that could be triggered by some combination of sprinting/toggling underbarrel attachments/toggling firemodes or something like that? The weapon it happened with was the FAL, and my friend usually played the SKS, so I believe that's where it happened for him.
I am sure it is not a hit registration issue because it is very obvious when the gun does not shoot at all and neither sound is played nor any of the vfx that accompany a shot.
It's a rare bug, but since it will usually end with you dying and raging at the game, I think it's worth looking into.
Hope this helps.