H&R will not fit a used barrel to your frame, they will only fit new barrels that they sell to your frame when you send it in. Some used barrels will fit fine, others won't, it's just a matter of owning enough frames to get one that fits good. If you only have one frame, odds are not the best that a used barrel will fit. Any good smith should be able to fit a barrel for you, just make sure you are fitting a barrel that is suitable for the type of frame you have, IE, not rifle calibers on shotgun frames, etc. If you use the search feature in the NEF Centerfire forum for barrel fitting, you should find what you need for a DIY fitting.