Old thread, yada yada, but I've recently found a fantastic company by the name of Original SOE Gear making fantastic products.
If you have the need to carry a shotgun for a living or for protection, this small setup seems AWESOME. With this rig you could easily hold up to 24 extra shells on your support side, and still have a general-purpose/admin/dump pouch on your reaction side.
http://www.origina...com/12gamicro.html It's not cheap, but that's because it's not CTD Chinese bullshit made for airsoft. It's made for, and used in, combat.
They make a bunch of other awesome rigs. I'd particularly like the Micro M4 rig.