8 gauge industrial magnum shell with a 3 oz(1312.5 grains)slug has a typical muzzle velocity of about 1744 feet per second and a muzzle energy of about 8,863 foot pounds.
http://www.ballisticproducts.com/ 8ga wads
Ballistic Products Inc. 0000 Super Buckshot about 18rds in a 3.25in
built a 30lb 36in barrel 8ga single shot a few years ago from a nef 12ga parts and scrap metal and an .84in ID 1.5in OD pipe in a 14in long sleeve
had to make a stronger lower receiver moved the serial number
3in silicon buttpad on a fiberglass stock
it was heavy and ugly but worked
recoil and noise was worse than a 50bmg
no1 want to be near you at the range
too heavy for hunting and not legal for waterfowl in the US
slugs kicked much worse than buckshot
NEF 12ga $70 other parts $100 sold for $250 to a friend
wasnt worth the time building i did learn to weld better
8ga 3.25in ind mag ammo is hard to find and not cheap
remington SP8 MAG 3 Oz. Lead was what i start with
wish building a 50bmg was in my budget