[blodia]

hitscan weapons use tracerays to determine if they hit clients, shouldcollide is fired for trace rays aswell. in css if you print debug messages for whenever shouldcollide fires you'll notice it fires when you look at other players aswell, this is the traceray for showing the players name when you have your crosshair over them.
your best bet is to see if the hitscan weapons all use a unique collisiongroup and contentsmask so you can tell if shouldcollide is firing because of damage. if this is for antistick then it would be easier to see if its just 2 players colliding, i think the collisiongroup was always 8 or something.