The "Light Weight Forward Area Air Device Unit" is actually a 37MM flair launcher attachment for Many military rifles such as AR15, M16, M4, AK47,OR COBRAY M11/9. Customized for the movie with its own stock that the sheriff assembles it onto before he uses it. It is not a grenade launcher, just a flair launcher made to attach to a rifle.
Sheriff Keough's "big gun" was a fake gun made specifically as a prop for this movie. They even gave it a fake name, calling it a "Light Weight Forward Area Air Device Unit" which itself is a pretty good parody of military jargon. It was not an M-203 grenade launcher, which fires 40mm grenades. The "grenades" shown with the gun were a good deal smaller than 40mm.
When Wells shoots the tranquilizer into the trapped croc you can see the pink-feathered tranq projectile. The second croc jumps, is shot, and we cut back to the first croc who no longer has the tranquilizer dart.
When the four main characters first canoe down the lake, an above shot shows the four properly rowing by holding the oars by the tops of the handles. In the following closeup, Kelly is holding the oars differently.