It could be the scope and mounts but....... The 1300 is called a speed pump for a reason. The rotating bolt headdoes not lock up the same way other shotguns do. When you pull the trigger it releases the bolt head and allows it to move rearward. If you are not holding on to the forearm when you fire the gunthen you are allowing the bolt and forearm to slam very hard into the rear of the receiver.

The reason behind this is to allow the shooter to cycle the gun faster than many other pump action shotguns on the market.

You must hold on to the forearm when firing a Winchester 1300.