Old rifle nut - you can still get most excellent barrels from PacNor, already contoured for right around $200 for chromemoly and $220 for stainless. I get my chambering reamers from Pacific Tool and Gauge - $87.0 for solid pilot and $150 or so for removable pilots.
; I've used the 'collar-method' of putting barrels in an action and it works well. Due to the small calibres I used, a throgh-hole of 3/4x16 worked well, but I can see doing this with a thin-shell threade stub in the action and replaceable barrels as you suggest. It would be fairly simple job for anyone with a lathe and who makes good threads. I have several switch-barrel guns - fun- fun.