Really? it would take you all day to make it? What kind of lathe does your pal have, a treadle lathe?
Seriously tho, if you are looking for a .700"ish straight mandrel to true the receiver from , you are doing it backwards.
Make a mandrel big enough to cut barrel threads on. Make the threads long enough so the mandrel bottoms out on the lug abutments. Screw the receiver on the mandrel(while it is still in the lathe) and then cut the receiver face from the back side. This will assure that the receiver face is square with the threads.