The Extreme is a really nice rifle but i would get a sample of Black Crushribs before buying any. They were way too loose in mine especially when combined with a .451 bullet such as some Barnes, Parkers or Scorpion PTs.

The HBCR sabot is around .503 with those bullets and a Knight 50cal barrel can run as high as .502 Even a .501 bore would be really loose with that sabot.