Actually, I know what's going on. When mossbergs are fired, their actions remain locked and closed throughout the recoil impulse. Now as the gun is moving backwards during recoil, it's also compressing the rubber recoil pad and then compressing your meaty shoulder and pushing you back a bit.
As soon as the recoil ends, both you and the shotgun move forward as you return to the firing position you had before the shot - now this happens in two stages. First is the elasticity of your body and of the recoil pad. This stage is pretty impossible to see, as it's going as fast as a stretched elastic tissue can return to it's shape before the recoil impulse. Second is just you moving back to the proper place, this happens slowly.
The first stage of post-recoil movement is pushing the gun forwards very quickly. Now as the frame of the gun is moving forwards, the static momentum of both the forend, action bars and action (bolt/bolt slide) is trying to keep itself in place (as it has come to a stop immediately after recoil, just when you start moving forward) and is going to stay in place until you either pull it forwards, or until the friction of the parts overcome the non-moving parts and the whole thing starts moving forward jsut like the rest of the gun.
Heavier loads pronounce this.
If you shoot a Mossberg one handed like a pistol and punch the gun forward as it's trying to recoil back, you can get them to eject spent shells.
This is all safe, as the pressure has dropped LONG BEFORE the action can be opened *fixed*
