Naughty move: this top hatch takes precious time. Imagine that you are SUDDENLY engaged by an enemy without any opportunity to cover and you have only a couple of seconds to shoot and an empty shotgun. If you're armed with standart Remington 870, it's not problem: you just put the shell in, cock it by pulling the pump and shoot. In your variant you have to open the hatch first, put the cartridge in, close hatch, cock and only THEN you may start shooting.
You can get the same effect simply by using a conventional detachable box magazine, like [link] without having to sacrifice accuracy or risk feeding problems. Shotshells are made out of plastic or waxed cardboard, they're more fragile than bullets. A single deformed one might obstruct the magazine and require the user to take the gun apart instead of just cycling manually like with a pistol or rifle.
It doesn't, since you'd still be dead, but that's kind of why it was sort of a silly scenario in the first place IMHO. If you're caught by an enemy out of cover with an empty gun, you're dead. Even with a semiauto pistol which can be reloaded in a second with lots of practice, that's enough time to get shot 3-5 times with another pistol and get mowed down by any automatic firearm.
This is why this isn't an issue in many open-bolt machineguns which in fact do load this way (PKM, M249, M2). Wouldn't help either way. Quite frankly, if you're "suddenly engaged" by people in a clearing within the 25-50 meter shotgun range, with no forms of cover, and you didn't see or hear them coming, you don't need a better gun, what you need is a new set of ears and eyeballs.
Hmm... Very intriguing... This design looks great, how much thought did you put into it? Also, Truvelo Armoury out of South Africa made a shotgun that loads a little like this one you're made [link] Maybe you should take a look it and compare to the two. What you learn might amaze you!