On one I completed recently the purchased parts were the RCM barrel and locking peice. I used the shortened carrier by Lee tool but there are longer carriers out there that may work better. (The extreme shortness requires a few adjustments elsewhere is all) Cocking tube was custom made from a blank from HK parts, I needed to use a HK33 cocking support that I cut apart, shortened then rewelded to work with the shortened Lee tool carrier. The recoil/driving spring typically coils up in the carrier so you will need a collapsable recoil guide rod since you no longer have as much room for it to do that. In the case of the Lee tool carrier mine would bottom out on the compressed spring before it got to the buffer. I made a spacer to move the buffer forward, cut off the bottom tab on the buffer so the buffer would clear the reciever reinforcement.
Its a great build and a lot of fun. My kids love shooting it and on a cold day the heat of the muzzle flash keeps your face warm. Good luck with this. Mine runs like a clock, took about 5 rounds for the build to settle/break in and now she runs no issues.
Basic point is most parts for this build are custom and not off the shelf.