So I was watching the Halo 5 (is it 5 now? I've lost count) E3 trailer last night and noticed that the main character was using a top-loading shotgun. It basically looked like it operated like an upside down pump action, with the loading port/feed tube on the top, and barrel/action on the bottom.

Anybody know of a reason why shotguns aren't made this way (other than increased mechanical complexity)? A quick google search didn't really come up with anything satisfactory.

Just curious to know if you guys have seen or heard of anything like this. Seems like a crazy three-gun guy would have developed a system like this already...