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.
Betty White's character is told that PETA (People for the Ethical Treatment of Animals) would be interested to learn of her alleged mistreatment of her cows. In reality, Betty White is a major on-air spokeswoman for PETA.
The "Light Weight Forward Area Air Device Unit" is actually a 37MM flare 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 flare launcher made to attach to a rifle.
The trivia items below may give away important plot points.
The size of the crocodiles in the movie is actually much larger than anything recorded. The current largest crocodile in captivity (Guinness book of records, 2015) is 5.48 meters (about eighteen feet), and although there are (unverified) reports of sightings over twenty feet in length, nothing is as large as the thirty feet quoted in the movie
This movie has similarities to The X-Files (1993) season three, episode twenty-two, "Quagmire", where a series of attacks happen on a lake, where it is thought to be a sea monster called "Big Blue", but turns out to be a crocodile. Other similarities include the farmer's cow being killed while drinking from the water, and a diver being killed.