Consider what happens when you test a handload. You hear it go bang, you
feel the recoil, you see where it hits the target, and you can measure the
velocity. We normally don't measure the intensity
of
the bang
or
the force
of
the recoil; we measure only the target and the velocity. Most important
is
where the bullet hits the target.
If
all the bullets go into the same bole, and
the average velocity
is
sufficient, you don't worry about velocity unifonnity.
If
the group
is
larger than you want, you grasp at anything that will give you
a clue
of
what went wrong.
The secret for making smaller groups
is
unifonnity. Other things being
equal, the more uniform you can make the ammo, the more likely it will
shoot to the same hole.
Uniform velocities are simply another indicator
of
uniform ammo. Uniform velocities do not guarantee small groups, nor do
large variations guarantee large groups. There are no guarantees, but you
can at least put the odds on your side. When you have uniform velocities
you can assume that you have a proper primer for the powder, that you have
a reasonable powder for the case and bullet, you did a good job measuring
the power, and that your cases were
of
uniform capacity. Uniform velocities
tell you very little about bullet quality, the bedding
of
the action and barrel,
or
if
the gun vibrations induced by the firing just happen
to
fall
in
a sweet
spot
When you have erratic velocities and small groups, your bedding is
probably good and you have a good average velocity for that powderlbullet
. combination, but be suspicious
of
your primer choice and firing pin.
[f
you
get both erratic velocities and large groups, go ahead and make significant
changes in bullet, powder type, or gun; you probably aren't close to any
perfect combination.
The common limitation on the formal use
of
standard deviation and other
statistical procedures
in
shooting is the number
of
shots required.
Statisticians call it sample size. Invariably statisticians ask for more shots
than shooters want
to fire. Shooters want to shoot five-shot groups, and
statisticians want to see at least twenty-shot samples. Firing more shots into
a group will give you a better statistical measure
of
both the accuracy and the
standard deviation.
Page 22
Trying to measure the velocity uniformity
of
your ammo by chronographing
only five shots
is
like measuring the accuracy with one five-shot group. One
group is an indication, but you can't trust it to repeat. Likewise, one standard
deviation number should be considered only as an indication
of
unifonnity.
Although standard deviation
is
the best available measure
of
velocity
uniformity, it
is
not good enough to be considered the only measure
of
ammo
quality.
Use standard deviation numbers as indicators
of
unifonnity, but use
them along with other indicators
of
load performance.
Do you always need to usc a large sample size
as
you are developing your
loads? When I shoot a large group, backed up by large standard deviation,
r
don't waste time trying to measure just how bad the load
is.
The load could
eventually prove to be a good one, but the odds are against
it.
I'm looking
for good loads, and abandon bad loads
as
quickly
as
possible. With the large
standard deviation confirming the large group, I abandon the load quickly
and don't feel guilty for shooting fewer than twenty shots. It's fun to shoot a
new load that
I hope is good; it's drudgery to shoot a load which I expect to
be bad.
Sample size takes care
of
itself with good loads.
If
a load looks good, even
though you've fired only one five-shot group, you don't abandon it and you
don't immediately accept
it.
You try at least two or three more groups
of
this
load to see
if
it
is
golden. The original sample
of
five shots is now fifteen
or
twenty shots.
[f
the load continues to look good, you load and shoot it still
more. Even a statistician would be happy with the total number
of
shots
fired with your
"keeper" loads.
What are reasonable values for standard deviation? What
's
a good group
size or a good average velocity?
It
all depends on what you're trying to do.
You should use the numbers only for comparison, and you don't compare
apples and oranges.
If
you're working
up
an
elk load for a .375, comparisons
to the performance
of
your ,45 ACP or varmint rifle are irrelevant. You
don't compare the average velocity
of
your .375 to the average velocity
of
your ,45 ACP. You don't compare groups from your .375 to the groups from
your varmint rifle. Likewise, you don't compare standard deviations
between your .375 and the other guns. The only comparisons that matter are
those you make between the .375 loads you keep and the .375 loads that you
abandon.
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