Turn down to fit into smaller hole. Mill through receiver and into barrel for two locking screws. Take out barrel. Drill holes to thread size and tap the holes. Use socket head, dog point set screws. These are all grade eight if inch, USA made screws. I like unbrakeo screws. Get black ones, not stainless.
Close tight tolerances on all your cuts. Hole and barrel diameters should be within .001 or less. Preferably +,- .0005.
Other ways but this is an easy quick change system. You could do it in two steps, as milling your holes on barrel separately than receiver just have to make sure you get your hole to hole dimensions exactly right.