Hello Fred: D
The hole is to have an air inlet, to propel the balls, on some there are even two !!
On a ball shot, in principle the barrel is forced (see glued?) Into the mechanism, because no recoil, on the other hand there may be a locking system by a plastic part? I just looked on a KT ball shot (metal barrel) from Torro, and it's done like this, on the other hand I have a flat on the biggest part of the barrel level mantelet, but no trace of screwing

remember how I did it? But don't remember having had a hard time

!!! For me, everything must come out from the rear, seeing that in all the photos, the mask remains on the turret?
Sorry for my spelling mistakes, I try to do better every day...