From the rear of the rifle on the right side of the lower stock there should be a pin with a rounded head that retains the barrel band on the rifle configuration. If that pin is worn down and or pushed in to far it will allow the band to move when firing. The presence ofg the pin may be obscured by finish and hard to see but if the stock is original it is there. The solution IMO, is to replace or push the pin out from the opposite side until it once again gives purchase to the barrel band. I would not recommend over tightening the band sans the sling swivel as it might put undue pressure on the stock and fracture it. The carbine stocks used a barrel band retention spring to accomplish the same thing as the pin did on the rifles. Good luck