big deal about head space in assembling a new upper ???
stop me if i am wrong, there is only one way to insert the barrel into the bare upper, right ? you can grip the barrel wyth special vise jaws or a receiver clamp which also fits into a vise, right ? you then tighten the barrel nut assy. till it is very "snug",(abt. 20 ftlbs) then tighten till the notches for the gas tube aligns,(abt. 35 ftlbs) that is usually not more than one or two notches past "very snug", drop in a fully assembled new bolt carrier wyth new bolt, place an unfired round in the chamber & push the BC forward wyth thumb, bolt shud lock up on cartridge wyth a small force, thats good !! if it takes stiff resistance wyth thumb to lock it, thats OK, the head space will "wear in" wyth "break in", the head space is only "BAD" if you can not lock the bolt on an unfired round wyth thumb pressure alone.., this requires a chamber reamer fix !! or return the barrel.......
just keep in mind that if your bolt does NOT lock up on a factory made round, you got problems !!!!!!!!!!!