there's the likes of the mad max sawnoff shotguns, but that's limited to 2 shots and a finnicky reload.
tbh as ak47frizzle says a compact smg/machine pistol is going to be your best bet, mp5k, mp7/9, an aeg would increase round count over a gbb (mp7 gbb isn't going to offer much over an extended mag glock) which would result in a much higher effective amount of fire.
tbh i suspect real shield tactics revolve more around one man with the shield and his buddies using him as mobile cover rather than a video game esque run and gun tank, i can speak from experience (local site used to have a couple of full size "riot" shields) and i can assure you it's damned awkward to fire and use it as cover (and it isn't as much cover as you'd think it is). most folks who do it effectively use it in the fashion of the old crossbowmen and drop the shield on the ground, lean it on their body and use their rifle 2 handed.
bearing in mind these are just cut up bits of plastic oil tank let alone actual steel and they were damned heavy, we did a juggernaut game where i was the "juggernaut" with the shield and could be tagged in by either team to fight for them but was otherwise invunerable (ironically making the shield a superfluous marker for who i was) and after a 30 minute game my response was "**** that to ******* hell some other ****** can take it next round my arm's ******"
to add to it a mate of mine made a replica of the rainbow 6 one out of steel and so far i've seen it be used for all of 1 game.