Hornady Bullet Comparator - Manual (Page 3)

How to use: Attach Body to caliper blade with provided
Thumb Screw. The slot in the Comparator Body is
machined to allow either "off-center" or "on-center"
alignment with your caliper blade, depending on which of
the two threaded holes are employed.
When using the OAL (Over-All Length) Gauge and Bullet
Comparator in combination, always attach the Comparator
Body to the "traveling" blade - in the off-center position -
then measure using the configuration shown in the
illustration. This method will align the OAL Gauge parallel to
your caliper. For all other uses, the Comparator Body may
be attached to either blade in the "on-center" position.
Place Bushing Insert (corresponding to bullet diameter) into
Comparator Body. Prior to measuring, close the caliper
blades snugly on the Bushing Insert and Comparator Body,
and tighten the Thumb Screw and Set Screw. Then, "zero"
your caliper (to 1.000") by setting the dial to “0” to achieve
the same starting point in future measurements. To take a
reading, open the caliper blades, center the case-head (or
bullet base) on the caliper's fixed blade and slowly close the
caliper so that the bullet nose feeds into the Comparator
until contact is made. Be certain all surfaces are aligned
and all slack is removed as light pressure is applied to
caliper blades (be consistent).
When setting up your seating die, compare your results
(using measurements taken with the Bullet Comparator)
with those from the OAL Gauge or a "dummy round."
Remember that as you reduce the cartridge OAL from
maximum, the difference will be "free-travel" (jump) to the
rifling.
Reducing bullet free-travel to closely match the chamber
dimensions can provide significant improvements in
accuracy. However, for hunting rounds we recommend that
bullets are seated at least .020" off the rifling to ensure
reliability in the field.
Remember: different bullets have different ogive curvature
(i.e. #6 vs. #10 ogive, and secant vs. tangent ogive). As a
result, the bullet free-travel (jump) may change as much as
1/16" or more when one bullet model is substituted for
another. Therefore a comparator cannot be used to transfer
dimensions from bullet style "A" to bullet style "B", as this
would be an "apples and oranges" comparison (the same
reason the mic's - which employ an imitation bullet -
produce errant results). When changing bullet models, it is
imperative to get new cartridge OAL dimensions from
reloading reference material, or from the firearm itself using
a bullet seating depth gauge. The best gauge for this
purpose is Hornady's OAL Gauge.
Thumb
Screw
Set
Screw
OAL Gauge
(partially shown)
Traveling
Blade Body
Bushing
Insert
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to foster a better understanding of and greater
participation in the shooting sports.