I got to fire an 8 gauge gun - it was mounted on a tripod and shot at clinkers on the boiler tubing at a power plant - the ammo was HUGE and VERY expensive and not generally available to the public.
Added: IIRC, we actually LEASED the gun and bought the ammo from Winchester. They used to use Remingtons, but shooting slugs and buckshot tired folks out and wore out the guns