50 caliber lead ammo is great, but doesn't fit perfectly in any barrel AFAIK. For those using type L 1/2" copper pipe, you can buy sabots from mmpsabots.com that will allow you to shoot them with a perfect seal in the barrel.
They sell sabots ranging from 0.36" to 0.58", and they work *really* well when used in a barrel that they fit well in.
http://imgur.com/a/tkXo6
Just spreading the word about the existence of these nifty things
