Look at Cabelas Hawken, Traditions Hawken, CVA Hawken and T/C Hawkens. All have 28" barrels with 1:48 twist and can be ordered in side lock (percussion) or flint lock. All are legal in OR.
Depending on how much muzzleloader hunting you are going to do, start out with an affordable "smoke pole" that you can learn on.
FYI the faster the twist the better the gun will be for shooting conical lead (1:28"). The slower the twist (1:60") the better it will be with round ball. The Hawken with its 1:48" was intended to shoot both round ball and conicals well, but neither excellent. With some practice, just like center fire rifles, you will figure out what your muzzleloader likes to eat.
Good luck. Feel free to PM me if you want more info.