Mr. White (Harvey Keitel), Mr. Orange (Tim Roth), Mr. Pink (Steve Buscemi), and Mr. Blonde (Michael Madsen) were all armed with one Smith & Wesson 659 9mm pistol. They are distinguished from M1911s and other Smith & Wesson variants by their second generation features, stainless steel construction, and double stack frames. Though all of the guns look to be the same, some have the old squared trigger guards, and some have round ones.
Smith & Wesson 639
Though everyone is given a Smith & Wesson Model 659 for the robbery, Mr. White (Harvey Keitel) is seen using a Smith & Wesson 639 with Pachmayr grips. We can assume, being that he is a professional thief, that this is his personal gun, and not a continuity error in the film. During the getaway, he is seen dual wielding his personal Smith & Wesson 639 with his issued Smith & Wesson 659, further strengthening this theory. In the special edition DVD, on the second disk a deleted scene can be found which briefly shows Mr White's/Larry Dimmick's police record. This informs us that his weapon of choice is a Smith and Wesson 9mm, even further strengthening the personal 639 theory.
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Smith & Wesson Model 669
When Mr. White hits Mr. Pink, you can briefly see his gun holstered, though it has changed to a Smith & Wesson 669.
Beretta 950 Jetfire
Mr. Orange packs 2 handguns while getting ready to meet Nice Guy Eddie (Chris Penn) Mr. White, and Mr. Pink for a meeting with Joe Cabot (Lawrence Tierney). The automatic he holsters at his ankle is a Beretta 950 Jetfire in .25 ACP.
Charter Arms Off Duty
Mr. Orange's second gun is a Charter Arms Off Duty with Pachmayr grips. He checks to make sure the cylinder is loaded, swings the cylinder back in, and then places it in his coat pocket. Mr. Orange is also seen with this gun in a paddle holster in an earlier scene, so it is safe to assume that this is his police issued sidearm.
Colt Detective Special
After Mr. White kills the pursuing cops with his two pistols, he and Mr. Orange walk until they find a car driving by. They aim their guns at the driver, a woman, and attempt to take it. She produces a pre-1972 nickel plated Colt Detective Special from her glove compartment and shoots Mr. Orange in the stomach as he opens the door to pull her out of the car. Another Detective Special is "Mexican carried" by Joe (Lawrence Tierney), distinguished as a Detective Special by it's round edged butt.
Smith & Wesson Model 66 Snub Nose
In the final standoff between Eddie, Joe Cabot, and Mr. White, Eddie aims his Smith & Wesson Model 66 Snub Nose at Mr. White, threatening to kill him if he doesn't take his gun off of Joe.
Smith & Wesson Model 19
One of the police officers in the scene where Mr. Pink is being chased has a Smith & Wesson Model 19 revolver as his sidearm.