In the new Joker movie there are a lot of things that are explained (or can be explained as metal illness) but why Randal did say that Arthur tried to buy a gun from him?
It is in the scene where Arthur speaks with his boss (Hoyt) in a telephone booth.
I have in mind only one explanation, that Arthur really tried to buy a gun from Randall, but he refused so Arthur gets a gun some other way and just thought that it is from Randall.
But if it was "for real", why Randall would snitch and lie about Arthur? He seems friendly but his actions were possibly ill-intentioned.