hahaha...ahh the 'ol bazooka glitch. Don't worry freddy its harmless.
How to do it: Walk into shallow water with a zook kit. Fire the zook, and immediately step into the deeper water (so you guy starts to swim). Get out, and QUICKLY switch to colt. I have 2 extra mouse buttons mapped to primary/secondary weapons, but all you do is switch between the two real fast. All you're doing is not letting the animation finish (reload zook). Do this (from colt): Switch to zook, fire, switch to colt. Repeat...you can fill the sky with bazooka trails, lol.
NOTE: This is NOT a hack! The animation and zook effect play, but they do no damage.
Same with sniper rifles, but you don't need to go into water. Fire a shot from a full clip, then switch to colt real fast. Do the switch/fire/switch thing, and it will SOUND like you have a rapid-fire rifle...but every bullet after the first does no damage.
That guy is a amateur, I'll post a vid later of filling the sky with zook trails lol