With a fully rifled barrel on a 12 or 20 you should be good to go out to 150 yards. Get a low-powered scope and try different brands and types of slugs to see which shoot the best in your gun. I personally use a Rem 870 with their rifled slug barrel and the Remington Copper Solid slugs. It's accurate enough and carries enough energy for that 150 yard shot. They don't shoot as well in my Benelli SBE slug barrel. My son shoots a Browning BPS 20 gauge with the Hornady 250 grain FTX sabot slug and that particular combination shoots well, too. You really have to try a few to see which will shoot well the gun you put together.
A muzzle loader will also kill easily at 150, if you hit with the one shot you get. I use Barnes 250 grain TMZ sabot slugs in a Lyman Hunter. In-lines are easier to work with and I can't scope my Lyman, but it is plenty accurate, too, even with the aperature sights on it.
Fast Ed