When Nick and Casey go into Ballpean's flat, nick is frisked by the bodyguard, who finds no weapon on him. Casey refuses to be searched. When a fight breaks out, Casey pulls out two pistols, so we understand how he smuggled them in, by refusing to be frisked. However, Nick pulls a machine gun out from under his jacket. There was no way he could have smuggled this in without the bodyguard catching him when he frisked him. It's a big gun too, which would have made it even more obvious if he tried to smuggle it in.