The 308 bushing is fine for measuring to a standard reference/diameter point on the ogive --
after establishing your desired standoff using the OAL tool. (or any other method you choose)
The bushing's diameter doesn't have to be "exact" (though it's pretty much so), since all you're
doing is measuring to the same reference diameter every time.
As to bore diameter, see here:
http://www.milsurps.com/showthread.p...l=1#post158154