A .45 TC Hawken will have a 15/16" barrel. Green Mountain makes (or made) some .54 caliber 15/16" drop in barrels for the Hawken, but they are hard to find. .50 caliber 1:70 twist and Long Range Hunter barrels (1:28 twist) are readily available in the $200-$225 range from either Green Mountain, Track of the Wolf or Midway USA. If you want to shoot sabot loads with that Hawken I can tell you both of my .50 LRH barrels shoot great with 250 grain Deep Curls and Harvester short black sabots. I'm sure the .50 1:70 will shoot patched balls very well. But if you can locate a .54 1:70 twist barrel in 15/16" grab it fast.