The thread shank on the Tikka barrek is some .200 longer than what is on a Ruger barrel. I have two ways of overcoming this. The best way would be to face the barrel off to the shank length of a normal Ruger barrel, chamfer it and then rechamber the barrel to correct headspace. The "other" way would be to screw the barrel into the action on a go headspace gauge pretty snug against the bolt, measure the distance between the shoulder of the barrel and the action, then make a spacer that dimension +.003, just for laughs maybe.