Primers backing out is usually an indication of very LOW pressure.

This will happen, for instance, when shooting primer propelled wax bullets or plastic practice bullets in a revolver.

Normally, recoil sets the cartridge case back against the breech face and irons the primer flat to the case head.

Check the contour of the primer after firing. If it's still round at the edges, the pressure is very low.

"The price of good shotgunnery is constant practice" - Fred Kimble