Elwood uses a blued Beretta 92SB as his sidearm, as noted by the rounded trigger guard. The county police are seen with 92SBs in their holsters as well. (They are assumed 92SBs because why buy another gun if you don't even see it?)
Dennis uses a Beretta 92SB with a stainless finish as his sidearm.
Browning Hi-Power
Elwood is seen briefly holstering a Browning Hi-Power with black grips, but it is never seen again.
Colt Single Action Army
Del Knox (Burt Reynolds) keeps two Colt Single Action Armys in cross draw holsters. He uses them to blow up Dennis and Elwood's ATV, stopping the assault. He fans them from both the left and right hands, which fire five shots each, a well-know safety precaution with old six-shooter revolvers (load five cartridges and drop the hammer on an empty chamber). They are only briefly seen and could be Uberti, Cimmarron, or Ruger Vaquero copies, or perhaps genuine Colts.
Shotguns
Mossberg 590
Elwood (Ethan Suplee) wields a Mossberg 590 in the scene where the boys stumble upon he and Dennis' pot field. He fires several shots, missing the fleeing boys.
Mossberg 500
Elwood (Ethan Suplee) trades his Mossberg 590 for the more traditional Mossberg 500 (which is seen again later in full, showing it has an extended magazine tube). Sheriff Briggs (Ray Baker) also uses a Mossberg 500 later, showing off its extended magazine tube, when he meets up with Elwood and Dennis at the end of the film.
Dennis (Abraham Benrubi) uses an L1A1 with a laser sight as his weapon of choice throughout the film. The inch pattern is evident by the front sight and the rear sight as well as details in the upper receiver and pattern of the foregrips. Rather than adapting the rifle for blanks (Hollywood style), an actual Australian military blank-Firing adaptor used in training is attached to the rifle. This milspec blank adapter is usually painted bright red when used with military blanks, however, the filmmakers painted it black, likely to blend in more with the rifle. This also means that they could only use military surplus 7.62x51mm blanks, and not the super hot Hollywood style blanks, as it would damage the blank adaptor.
M16A1 w/ Cobray CM203 37mm Flare Launcher
Elwood later uses an M16A1 fitted with a Cobray 37mm Launcher. Towards the end of the film, Dan (Seth Green) picks up Elwood's M16A1 but shows his lack of experience as it fires wildly. In several shots it appears that the bolt carrier has actually been removed from the rifle. This is a quick and easy method to render the rifle completely safe when in the hands of an untrained actor. In several unfiring scenes (and some quick cuts when the rifle has stopped firing) the viewer can see the rifle, sans bolt and bolt carrier. However, this is not commonly done because sharp eyed viewers will wonder where the bolt went.
Machine Guns
M60
Elwood loads up an M60 machine gun and uses it with great effect to destroy most of Del Knox's (Burt Reynolds) house.
Other
Fake Mk 2 Grenade (Cigarette Lighter)
In a very strange directorial decision, a pewter Mk 2 hand grenade cigarette lighter is used by Elwood (but presented as a REAL grenade), who drops it when Dan knocks him and Dennis into the mine shaft. Jerry (Matthew Lillard) picks it up and manages to luckily take out the three bad guys when the grenade blows up a tree, falling on them just in time. Dummy Mk 2 grenades are plentiful, so it is somewhat mysterious as to why they went out of their way to use a cigarette lighter novelty item.