Keep in mind that the bolt head does lock into the barrel and remains locked until you pull the trigger and all of this happens in miliseconds.

It rotates and locks much like a bolt action rifle but as soon as the hammer reaches a certain point it releases the bolt. This is (was)why the 1300 is marketed as the speed pump. The bolt head unlocks and the recoil of the gun helped the shooter cycle the pump faster than on other pump shotguns.

When you do not hold on to the forearm on a 1300the bolt and forearm will slam into the rear of the receiver.