HB-9 updated text (PDF) - Corbin Bullet Swaging

inches, you have some chance of telling it is between 0.3085 and 0.3075
inches. If you buy a gauge block with guaranteed traceable dimension of
0.3080 inches plus or minus 0.0001 inches, you can set your mike as a
comparator to see if your bullet is .3081 to .3079 inches.
But you can’t tell if you have a .30805 or a .30795 inch bullet. A
screw thread measuring system just won’t repeat any closer than that. All
it can do is give you a readout where the numbers themselves are guaranteed
to be whatever the ad says, not that they represent what the measured
part actually is. That is how many precision tool brands are sold to
the public today. The difference between readout and instrument precision
is just complex enough so that some people don’t care to understand
it. It’s much easier to believe you bought the precision of a $5,000 lab
tool for $150.
What matters is that the bullets land in the same hole, or as close to it
as possible, and there is no way yet devised to determine if they will do
that before shooting them! Bullets that are undersized to the bore by even
half a thousandth may show signs of lower accuracy, whereas bullets that
are a thousandth oversized from this ideal usually shoot as well as the
“right” size bullet.
But we don’t really know in advance what is “right” because it depends
on your particular barrel. We do know that in general, if you want
maximum accuracy, you should strive for a tolerance range of minus
zero, plus one thousandths of an inch from the groove-to-groove depth.
You may not find this is “right” for every gun or load, but it is a good
starting point.
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