Raiders of the Lost Ark

The following guns can be seen in Raiders of the Lost Ark:

RPG-2

Near the climax of the film, Indiana Jones (Harrison Ford) points a shoulder-fired anti-tank weapon at the ark while it is being transported by the Nazis and threatens to blow it up. The weapon is actually a Chinese Type 56 copy of the Soviet RPG-2 made to look like a WWII-era German anti-tank weapon (it features a Panzerschreck-style shoulder grip). Since neither the Panzerschreck nor the Panzerfaust existed in 1936 (when the movie takes place), he should not have a weapon like this.

RPG-2

Indiana Jones with the "German" RPG-2.

Another view of Indy pointing his RPG-2.

Nordenfelt gun

A twin barrel Nordenfelt gun is seen mounted at the rear turret of the 'Flying Wing' plane.

MP40

Many of the Nazis use MP40 submachine guns in this movie, which is not appropriate since the film is set in 1936 and neither the MP40 nor its more expensive predecessor, the MP38, existed at the time.

Walther P38

Used by various Nazis throughout the film. A 9mm Parabellum 8 round pistol not produced until 1938 and as such an anachronism in the film.