I'm sending mine In to have the factory do it. I have it on good authority that the factory does NOT use shims. They do what they do, and the result is virtually "factory new"..... That's what I want. I don't want barrels with metal shims glued on. So, I'll be boxing my gun, and the three barrels up, and shipping that off to H&R for their magic. I'll report back on the fitting when I get the gun and barrels back.
BTW, addressing an earlier question, the instructions on the H&R site strongly suggest that you NOT MARK the nature of the contents on the box.