My Cousin Vinny : Murder weapon

I know there wouldn't be a movie, but what about the murder weapon? Didn't they see they didn't carry a gun and because tey were caught not so far away from the crime scene it should be somewhere in the road if they discarded it soon after the murder.
The prosecutor says something at one point, but shouldn't it be an important issue?

Some unknown accomplice after the fact could have discarded it for them. As one theory.

Yes it should've been a more important issue and brought up more by the defense. A accomplice would be unlikely since they were from New York so they would of had to of thrown the gun away at some point between the robbery and being caught. You think they'd search the road, garbage cans, rivers, lakes, sewers, etc. on the route between where the robbery happened and where they were caught. But they also had a "confession" so maybe wouldn't of looked (although they would at least ask what they did with the gun) and like you said it's a movie.

That's actually a good point. I know we aren't given an actual time frame for how long had passed between them leaving the store, the murder going down and then the deputy tailing them and arresting them, but I'd imagine the Sheriff's Department probably figured they drove for 10 minutes, got out of the car and then hid/burried the gun somewhere off the main road. They probably tried to find a gun but when coming up empty they just assumed they missed it rather than it was never there.

Another thing; at the end Vinny says "what caliber gun was used to murder the clerk?" And the sheriff replies "a .357 magnum".
They wouldn't have know that a .357 magnum was used in the murder. They only could have know that a .38 caliber slug was removed from his body. (Yes .357 magnums shoot .38 caliber bullets)

Actually a .357 Magnum is a calibre. It's like saying a 9mm Luther (a very popular 9mm round). Magnum makes ammunition as well as guns, much like how Smith & Wessson make the S&W .40 cal and you get all sorts of guns that are chambered in that (for example the Glock 22 and 23 and the Beretta 96, 90two and Px4).