Mossberg 590 or 870 police model for defense is about as powerful as it gets for a good American made shotgun.
One of these types can be picked up cheap at a gun store and don't forget the Ithaca 37 or the Model 97. The Mossy or 870 can be gotten dirt cheap and Century has police trade ins quite often for very cheap. If you know an FFL or dealer willing to get one for you then it's all good.
The other thing about a Mossy or 870 is getting a hunting barrel for cheap later on to use the gun as a multi purpose shooter.
One of these types can be picked up cheap at a gun store and don't forget the Ithaca 37 or the Model 97. The Mossy or 870 can be gotten dirt cheap and Century has police trade ins quite often for very cheap. If you know an FFL or dealer willing to get one for you then it's all good.
The other thing about a Mossy or 870 is getting a hunting barrel for cheap later on to use the gun as a multi purpose shooter.