"Smallest group size or your money back."

I've seen where people recommended chucking the mandrel in a drill press and sanding it down a thou or so... always seemed like getting a consistent diameter over the length would be problematic.
Monte,

I have quite a supply of mandrels for my .308W donated by the incautious who can be bothered reading instructions. I've had no truble slimming them down in my 3/8" chucked drill press, which just happens to have the jaws relieved at exactly the right place to slip the retaining cap into. I've found that 600 W & D run up & down in a regular motion will take off a thou without too much trouble & that a quick spin with 1000 grit will give it a mirror finish. I now have mandrels .001", .002" & .003" under fatory size for particular applications.

I also adjust my die according to John Valentine's procedure.
 
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