First you did not say whether it is a .50cal or .54Cal. After that you can guess that it is the normal twist for that caliber but it is rather easy to use a ramrod that the end is pretty solid with a patch that will allow the rod to follow the rifling and just mark a spot on the ramrod with the rod bottomed out in the barrel and pull it up measuring a half turn and then double the length which will be your length of one full turn(one half turn in 24 inches would result in one full turn in 48 inches or 1 in 48 as an example). Most of the T/C Hawkens will shoot the Maxiball or Maxihunters with reasonable accuracy up to 100 yards. Personally I would use the heaviest I could shoot and try not to overpower them. You also did not say where the rifle came from. Did you buy it new, used etc? There are barrels that fit the rifle available from aftermarket sources so if you did not buy it new it might have an aftermarket barrel with any twist the original owner decided to put on it. Measuring it yourself it really the only way to be sure.