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

Mixing bullets that have eccentric weight variations into a group that
has none will increase the group size. Mixing thin walled concentric jackets
with thicker ones can change the group size only because the friction
of the jackets as they pass through the bore may be different, so the powder
burns a little differently, and the velocity may vary. This can cause the
bullets to drop more or less depending on their velocity. The variation
due only to difference in weight, meaning the gravitational drop, is so
slight at 100 yards on a few grains (such as 2% or so of the bullet’s weight)
that you can disregard it. You may as well talk about the effects of an
airplane flying over and its gravitational pull shifting the bullet impact as
the weight variation in a 2% or less situation.
If you make your own bullets, and you have jackets that not only
weigh the same but have walls that are the same on all sides, and you seat
the lead cores to the same pressure so there is no loose core and no air
pockets, then you will be able to ignore weight variations of less than 1%
of total weight for any kind of shooting, and below 2% for anything but
top level benchrest competition. Any weight variation in this range would
be simply more or less core, concentric to the bullet center line, and would
have no serious or noticeable effect on group size.
If you have the same weight variation and it can be shown that the
cause is eccentric walls or anything else that causes the weight to be
shifted in an eccentric manner, then you will probably notice an increase
in group size. So, weight variation is not an absolute measure of quality,
but it is an indicator of a possible problem.
Eccentric bullets are easy to make by putting a known weight of nylon
string down one side of the jacket before swaging in the core—you
can control the weight and position of the variation this way. We find the
groups of bullets made with concentric weight variation (more or less
core weight) are within the average size for the control bullets, whereas
the eccentrics tend to fall outside in proportion to the amount of eccentricity.
My point is that weight is not some absolute number that tells you
“good” or “bad” about a bullet. After all, a 2 grain plus or minus variation
on a 50 grain .224 is plus or minus 4% of the total weight and may have
some noticeable effect, whereas the same variation on a 500 grain .458
bullet is only 0.4% and is below the limit of accuracy of most electronic
meters and chronographs, and is unlikely to have any affect that can be
measured.
Get out the instruction manual for any high precision digital scale,
and you will probably find a “specifications” section that tells you the
accuracy of the scale is from 1% to 5% of full scale reading, plus or minus
the final digit. That is, if you are using a scale that goes to 500 grains, and
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