Indiana Jones is fighting with the Germans in Marion's bar. He pulls out his revolver and starts shooting. As scenes cut from him to the germans and back, his weapon switches back and forth from a revolver to an automatic pistol. In some shots you can even see the spent shells ejecting as he fires.
The revolver is his main weapon, but the automatic is his backup weapon. There is some debate about where he carries it, but it is widely believed that he carries it in his satchel. If you watch the scene, his revolver runs out and you can see him start to reach toward his satchel, though that is also where his holster is. The pistol is a Browning Hi-Power 9mm semi-automatic and it is very similar to the Colt M1911. Production began on the Browning HP in 1935 by the Belgian company FN. This semi-auto can also be seen before Indy boards Katanga's ship. While you all have keen eyes to notice the weapon switch, this is definitely NOT a slip-up. There are actually three types of firearms that Indy carries during the movie. In addition to the semi-auto, the revolver is represented by two distinct models posing as the same pistol throughout the movie.
If you watch closely you can see Indy start to put his revolver back in its holder, before they cut away. Maybe he had two guns. Anyway "you know what a cautious guy he is".
It isn't a Luger - they have a distinct "chunky" shape, I think it's a .45. None of the German agents have Lugers, they all have Mauser machine pistols or MP40s. The only one with a pistol is Toht (the guy with the glasses) and he has a Walther P38. Anyway how would Indy get hold of one?
On an interesting side note: Neither the MP40 submachine gun or the P38 had been invented by 1936 so I don't have a clue how they ended up with them in this movie.