The Firearm Salesman or 'Mobile Gun Show' is a Trader that can be found in Trader Camps. His appearance can vary but he used to always have an army helmet and a Minigun. But not anymore.(i think)
By default he will only sell basic weaponry but can be convinced to show 'The good stuff' by a character with high Shooting if the party has over 11 Food.
He cannot be robbed or told to 'Cool it!'
The Group finds a man selling a bunch of firearms. He's holding a minigun that has both a scope, a laser sight, and a flashlight attached to it.
However, his selection seems like pretty common stuff.