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 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.