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 spokesperson 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 metres (about 18 ft), and although there are (unverified) reports of sightings over 20 feet in length, nothing is as large as the 30ft quoted in the movie