Barrel appears to be walking out?

[DonVoigt]

I'm with Doubs on this one, someone likely "removed" the receiver threads and "installed" by pressing or ?? a modified revolver barrel(.38 special or .357 = 9mm).

I believe you would best be served by purchasing a barreled receiver(upper) and retiring the one shown.

I believe the "pin" was a set screw, one can see the "dimple" in its remaining face where it was ground down.

Measure the diameter of the shank of the barrel that is showing, it should allow us to determine if there are threads left in the barrel or not. If it measures any larger than 18.0 mm or 0.71", then the threads have been removed or reduced.

[sheepherder]

Originally Posted by DonVoigt

I believe you would best be served by purchasing a barreled receiver(upper) and retiring the one shown.

Yes on the new upper for center-fire shooting; but this upper could be re-configured for rimfire/.22 shooting.

It could have a plug silver-soldered/brazed in place and bored out to take a .22 replacement barrel. There have been several .22 toggle-trains for sale in the last year.