I know someone who can blast through the 14-round magazine of a Henry lever action rifle, very very quickly -- he can also blast off ten rounds out of M1886 lebel bolt-action, operating the bolt left handed, with incredible celerity as well. Let's just say he's very dedicated. :-)

I suspect there is something to the nature of size of the shells and the particulars of their shape that may mean a pump action is more reliable or robust. Armies generally preferred bolt action rifles over lever-action, so there is more concerned than sheer volume of fire. This is speculation, but I was under the impression that lever actions weren't typically used with heavy loads.

As for a belt fed automatic shotgun, you really have to ask why you want something belt fed in the first place, and are you willing or even able to handle the consequences.

I would imagine that you would want a belt when you need or desire continuous firing. The consequences are probably awkward bulk, weight, and/or feed issues. A drum can be used (such as on a WW1 parabellum mount, or sometimes on Mg34/42s), but they typically don't have as many rounds as a full belt. Also, *I think* it takes more time to load a belt, than to replace a magazine. So, while you can fire for longer, when you need to reload it will take a bit longer, and might be a more clunky process. Again, it's just kind of speculation based upon some general knowledge of belt fed guns.