I see no hard requirement for a barrel nut - yes, I read the "could". [I'd look real hard at what might need to be done to the rifle to accept a barrel nut setup - ask McMillan, who, I believe use to offer a .416 Barrett chambering and may even have some "old" barrels.] A new barrel, preferably threaded for the same brake that is on the .50 barrel (or get a second one and avoid swapping), chambered and fitted with Go and No-Go gauges (I prefer having both = yes, for both cartridges), and a simple, deep, scribe mark on each barrel matching a corresponding mark on the action (one mark on the action) and you should be gtg. Unless you have a need for the reamer, the smith might provide it - or get one and use it on future barrels. Dies are available and good brass is now also available (the RWS direct from Barrett - ask for it while you are seated - super expensive). Enjoy.