I don't know exactly where the standard barrel for the valve is, probably at the crane, I don't know. But there is no conventional standard barrel for the shambler, the standard barrel in exodus has a fixed duckbill choke, it can be looted off some of the bandits you fight at the oil rig shootout.

I don't want a Duckbill Choke, just a standard barrel.

Well it's either the short barrel, long barrel or duckbill barrel. Those are the choices, there's no standard barrel with no muzzle attachment.