First off, consider that I have never seen a happy .45-120 owner. Too much recoil. You could do as well with a .45-90 or .45-100. An additional problem is that you just cannot burn enuff powder, even in a 34" barrel to do the 120 justice, the .45-100 is about max on efficiency.

34" barrels are long enough that even very well lubricated bullets run out of lube before they exit the muzzle and you get leading near the muzzle. 32" is a practical limit.

Hope that helps.