RE: 20 guage or .12 guage slug gun?
Assuming you can group your shots within reason (say 6" absolute maximum), while practicing out to any distance in the world??? This is for you to figure out?
I don't think I'd want to go much more than 125-150 yards with any slug gun, just due to the drop, etc...
That's a heavy bullet to push a looooonnnnngggg distance.
I would much rather shoot, maybe 75-100 yards and feel safe and ethical???