After being attacked by the group of assassins in the mob, Insp. Callahan (Clint Eastwood) is inspired to take out his old pistol, an AMC Auto Mag 280 chambered in .44 AMP. When his Smith & Wesson Model 29 is kicked into a curb drain by Mick (Paul Drake), he replaces it with his Auto Mag for the remainder of the film. The production crew had a very difficult time aquiring this gun to the point where they had to contact the original designer, who had enough parts in his basement to build two guns for the film. The first was left live firing so Clint Eastwood could shoot it and accurately depict recoil while the second was converted to fire blanks for the film. The gun's serial numbers were "Clint1" and "Clint2".
Jennifer Spencer (Sondra Locke) uses a nickeled Colt Detective Special as her murder weapon.
Mossberg 500 w/ folding stock
Horace King (Albert Popwell) uses a Mossberg 500 shotgun (12 gauge), with a folding stock.
Later, we see Mick (Paul Drake) using the same shotgun after stealing it from Horace.
I believe that the shotgun held is actually a Smith & Wesson Model 3000 12-gauge pump. S&W was in the Police market with revolvers and shotguns in the 70s and early 80s. Mossberg later purchased the 3000 series from Smith & Wesson as they got out of the shotgun market. The front barrel retaining nut is visably larger than that found on the Model 500 series.
Update : I added some information on the Smith & Wesson Model 3000 under the Discussion button above. . . BC2009.