Matt, I can see what you mean about the unaccounted space in the action and the possibility of a section of concealed barrel with a loading access port which can be pulled out for loading. However, what I can't see is, once the barrel has been loaded and pushed back, why doesn't it fly forward again under air pressure when the gun is fired, so exposing the port and losing all pressure? I presume you would need some sort of latch to temporarily hold the barrel back during the firing cycle?