with the combat shotgun (N), I'm pretty sure you'll need to pump it (you'll still have infinite ammo, but the fire time will truly be firetime + pumptime or firetime+movetime).
I have nano-modded a double or super shotgun, and there is a problem - you'll only get 1 shell in per action when you're trying to fire 2. however, since the second blast is often not that good anyway it should be ok. You can also alternate dodge and fire to get both shells.
I don't think the Shambler is worth fighting in AoSg unless you have plasma/Jackhammer, one of the nano assemblies, or MAD/MSh traits. You will have a shell box from the Armory, use it.