A Colt Detective Special is being used in the first Saigon Russian Roulette matches that Nick (Christopher Walken) discover. The ejector rod shroud that is unique to the 3rd Generation CDS is visible on the revolver being used.
Smith & Wesson Model 36
When Michael (Robert De Niro) goes back to Saigon for Nick (Christopher Walken), the revovler being used in the Russian Roulette matches is now a Smith & Wesson Model 36. Since the Model 36 is a 5-shot revolver, the chances of shooting one's self is now 1/5, instead of a 1/6 chance (as with a 6-shot revolver).
Smith & Wesson Model 10 Nickel 2"
Following the wedding when the men get ready to go hunting, Stan (John Cazale) is seen pulling his Smith & Wesson Model 10 snub. What appears to a different firearm is seen from the latter part of the movie. See the quote in the fourth screencap. Axel takes a Smith & Wesson Model 10 revolver with 2" barrel out of Stan's waistband to show Michael. Later in the hunting cabin Stan threatens Axel with it, and Michael takes it from him and puts one round in the cyclinder, clicking it empty against Stan's head. (Trivia: DeNiro requested a live bullet to be used in this scene to heighten the sense of danger. John Cazale, who played Stan, did not protest, yet compulsively and understandably checked that the bullet was not next in the chamber for the scene.)
Smith & Wesson M&P
The Viet Cong soldiers order the POW's to play Russian Roulette with a Smith & Wesson M&P revolver. Michael (Robert De Niro) notably uses it when he asks the Vietnamese soldiers to load three bullets in the gun, which he then uses to kill three soldiers and uses their weapons to dispatch the rest. It's identified as a Pre-Model 10 M&P from its hammer configuration. That is what is known as a "Long-Action" M&P. The "Long Action" was phased out in 1948. Smith & Wesson did not start numbering the models until 1957. By then the "Short Action" with the re-designed hammer was standard on all models.
Unknown Pistol
Police at the Saigon Military Hospital have an unknown pistol in their holsters.
Toy Single Action Army
What appears to be a toy Single Action Army is held by a child in this scene.
Unknown Shotgun
A Shotgun is seen on the rifle rack in Michael's house.
Remington 760
The rifle on the gun rack in Michael's house is a Remington 760 pump rifle.
Remington 700 BDL
Remington 700 BDL rifles are used by the members of the hunting party. Michael (Robert De Niro) notably carries a left-handed model.
M16A1
M16A1 rifles are carried by Nick (Christopher Walken) and Steven (John Savage) while serving in Vietnam. Michael (Robert De Niro) notabley uses one to shoot a Vietnamese soldier after hitting him with a flamethrower. Many soldiers carry them as well when Michael returns to Saigon to look for Nick.
Norinco Type 56
Norinco Type 56 rifles are used by the Vietcong. Michael (Robert De Niro) and Nick (Christopher Walken) both take one from them during the Russian Roulette scene and use them to dispatch them all.
AK-47
We also see VietCong with older style Type III AK-47s.