C-More was contracted to make a shotgun module for the US Mil that fits underneath an M4.  It is barrel, not handguard mounted, has a detachable 5 round box magazine, ways just a hair over 2lbs, and would be pretty kick ass to have down there.  

URL to World Gund article and pics: http://world.guns.ru/shotgun/sh28-e.htm

I emailed them and asked them how I could get one, w/ a statement that I realized that this would be a NFA weapon.  The replied that it was only for sale to the Mil right now.

I am looking for something to somehow mount under my FN FS2000 bull pup when it comes in, or under my M4gery barrel, but I am hitting walls here.  Does anyone have any suggestions?

I think you're a little premature with this.

C-More isn't going to make that available to civvies until they do so for law enforcement.  When that happens and assuming it won't be direct sales through C-More, you'll be able to get one through an LE dealer or distributor, though through NFA channels.

As for the FN2000, until they actually come out, no one is going to know for sure how accessores will attach, what's going to fit, etc.  It will take some time for them to make the rounds and for aftermarket companies to develop accessores for it that won't be offered by FN, like a shotgun module.  For sure though, because it's a bullpup, you'll have a very hard time mounting a shotgun under it, even an NFA one.  There just won't be much room.  Take a look at the 40mm module for it, very abbreviated.  Even a 9-inch barrelled M203 would be too long.  A 12-gauge will need an unusually short barrel, probably 5 to 7 inches to keep it flush with the rifle's muzzle.

You are right about that NFA barrel being unusually short.  between the bolt behind the barrel, and the room for bolt travel behind the bolt you don't end up w/ a lot of room for a barrel to keep flush w/ the muzzel.  What I would like is a SXS double barrel since it has a less space consumptive action.  Either one that somehow mounts under a rifle and opens down for easy access to the chambers, or one of those rare double barrels that slide forward from the breech.  If I put one of the latter under the weapon, perhaps upside down, I could use it like a the old gernade launchers that slide forward.