Here's the front end of my Atlas BB gun.
As can be clearly seen, there is no way to remove this shot tube. It's in there for the duration.
Here's a closeup of the multi-ammo Atlas with the shot tube edge pointed out.
There's knurling on this shot tube. It was meant to be removed.
I don't think there is any question that this is a never-before identified variation of the Atlas BB gun. It was intended to shoot multiple types of ammo (darts, slugs, BBs) instead of just BBs.