"I think that this is a dart loading port. The slim piece of brass wire seen is probably part of a wire assembly that holds the shot tube in a stable centered position. The shot tube is then pulled out far enough to allow insertion of the dart into the rear of the shot tube. Tube is then pushed back in (brass assembly keeps the tube properly positioned) Turn the loading sleever. Shoot dart.
Bottom line: if this is an Atlas dart gun instead of a BB gun then it's something pretty special. Nobody knows these old BB guns better than Wes Powers and if he hasn't seen one then it is possibly the only one of it's kind."
I'm stumbling on terminology. In last pic by Fallsflight we see the outer shot tube, some kind of access port with moveable cover on shot tube, a vertical divider of some kind within access port, and on the left inside that access port a shiny brass area, the barrel? Can't see if there's an opening into barrel by that pic.
There's another piece in there that's vee-grooved on its end. Whatever that thing is with a vee groove, it's outside barrel, inside shot tube? I was thinking it might be something protruding from an opening into barrel there, maybe a dart inserted into barrel. Whatever the vee-grooved thing is, it could be much larger than the part of it that's visible because lighting & shadows are playing tricks in the picture.
I wonder why the shot tube end cap is removable when there's already an access port into side of shot sleeve? Of course it would be a lot easier to pour BB's in through the end with cap removed than into the access port. That does seem to indicate the access port is specifically for dart type projectiles. Not enough room there for fingers to insert a pellet or slug.
I'm mainly wondering why that inner part has the vee groove & what that part actually is? Can you give us any more description & information Fallsflight?