Are you abusing the hell out of Juggler? In this version of the game it gives you the next LOADED weapon from your backpack. It's fiddly and time consuming to do, but you can have the "backpack full of shotties" effect going.
While I'm not that experienced of a player, I find that trying that Unlimited Shotgun Works setup tends to run out of steam around Phobos level 4-7 on HMP or UV, as you start to need more room for ammo and medkits. At first having a dozen shotguns works out okay, but after that point I find I'm hard-pressed to keep 6 at once in my inventory, which isn't reliable for picking off half a dozen demons. On the other hand, Combat Shotguns start to turn up around that time, but that doesn't seem to be what you are thinking about w.r.t. MGr.
