TexasMac's Web Site
By Wayne McLerran
Updated 4/12/20

Before I get to the main subject of this article the reader should
understand the difference between firearm accuracy and precision.  
When shooting and measuring the resulting group spread most
shooters including myself tend to incorrectly refer to a small group
size as an indication of accuracy, i.e., it’s a very accurate bullet or
load.  What we correctly mean is it’s a very precise bullet or load.  
Simply put, the ability to hit the intended target is a measure of
accuracy.  The group size is a measure of precision.  I.e., in terms
of hitting a bull's-eye, accurately hitting the target means all the
hits are grouped around the bull’s-eye.  Precisely hitting a target
means all the hits are tightly grouped even
if they are far from the

So now with the above understanding let’s get to the main subject,
barrel vibration, which directly affects shooting precision not
accuracy.  If you’ve been shooting and reading shooting articles any
length of time you’ve no doubt run across the term “barrel
harmonics”.  Similar to when a guitar string is plucked; when a rifle
is fired the barrel vibrates along its length due to standing waves
collectively known as barrel harmonics.  There are many standing
waves that will fit in a rifle barrel.  The longest wave that fits is
called the fundamental or 1st harmonic. The next longest wave that
fits is the 2nd harmonic and so on.
Notice that to “fit”, the wave length of the 2nd harmonic must be
half the length of the first harmonic. The wave length of the third
harmonic must be a third the length of the first harmonic, and so on.  
The trick is to find the barrel locations where most or all the standing
waves cancel themselves out, referred to as “sweet spots” for this
article.  By the way, several techniques are used to minimize the
effects of barrel harmonics on the bullet as it exits the barrel
including changing the load configuration, “free floating” the forearm
from the barrel, using an adjustable weight at the end of the barrel
(Browning Boss system), etc.  But don’t confuse the term “sweet
spot” in this article with finding the sweet spot when “tuning” a rifle
or a load for a specific rifle.

Update 4/12/20
The example above, known as a closed boundary system, is not
technically correct.  A rifle barrel is better described as an open
boundary system with one open end so any standing waves end at the
muzzle in an antinode rather than a node.  Here’s a good short
YouTube video correctly illustrating how it works:

Cross sticks are used in BPCR matches to rest the forward portion of
the rifle when firing.  Although a good argument can be made for
resting a true free-floating forearm on the sticks nearly everyone
rests the barrel on cross sticks since many BPC rifle forearms cannot
easily be free floated.  Also, due to the barrel weight of typical BPC
rifles, resting the forearm on the sticks would put the balance point
at or in front of the sticks and negatively affect the stability of the
hold.  With the barrel resting on cross sticks a vibrating barrel will
tend to bounce off the sticks unless the barrel is resting at one of the
sweet spots.  Watch the barrels at a match when some of the rifles
are shot.  Some will bounce a good bit up off the sticks while others
will only slide back.  When
you’re shooting ask your “spotter” or
someone close to watch your barrel.  Although the barrel deflection
may be the same from shot-to-shot, a bouncing barrel can only have a
negative effect on shot precision.  The goal is to determine and mark
the sweet spot or resting point so the rifle only slides back due to
recoil.  So how does one determine the sweet spot?  There are
several techniques.  Depending on your rifle and barrel configuration,
some may work better than others.

Remember, barrel harmonics as earlier defined consists of various
standing waves.  The rifle can be fired or we can induce the standing
waves and use the results to identify the sweet spot.  By the way,
since none of my rifles have a full octagon barrel I have not verified
that the talcum powder technique below will work.
• Talcum powder:
If the rifle has a full octagon barrel, sprinkle talcum powder along the
top flat and fire the rifle off hand.  Look at the powder pattern, it
should be lined up like this >>>>>>>>>><<<<<<<<<<.  The spot where
the powder lines reverse is the sweet spot.  Of course very light or no
winds are necessary for this method to work.
• Measuring the barrel:
If the barrel has no taper and is either uniformly round or octagonal;
measure the distance from the front of the action to the muzzle,
divide by 4.  The result indicates approximately how far back the
sweet spot is from the muzzle.  This definitely worked on my Stevens
44 with a uniform round barrel which is used for 22 BPCRA.  The same
location was confirmed using the dancing wire and stethoscope
• Dancing wires:
Clamp the receiver in a vice, padded of course, with the barrel
horizontal (level) and hanging out in space.  Find a source of thin,
bare wire and cut it into pieces about 5" or 6" long.  The thinnest wire
you can find that is lightweight but will hold its shape when bent.  A
good source is the copper wire out of four-conductor telephone cable;
the stuff that’s stung throughout your house to all your telephones.

Bend the lengths of wire into U-shapes (like a fence staple) with long
legs.  The U-shape should be slightly wider than the barrel so the legs
are not tight on the barrel but long enough that they hang down and
have no tendency to roll off the barrel.  Place the wires along the
barrel about every inch.

Now start tapping the barrel just in front of the forearm to set up the
vibrations, just hard enough to make the wires start to dance.  As
they dance they will move up and down the barrel.  After a good bit
of tapping they should have collected into two or more groups.  These
are the vibration dead spots (sweet spots) of the barrel and
theoretically should be the best places to rest the barrel on the sticks.

I expected this technique to be the most accurate but based on the
results it was not.  I tried it on 3 rifles as displayed in the following
photos.  It worked fine and agreed with the other techniques used on
the Stevens 44.  The location results were ¾” off the stethoscope
method on the Sharps and off by 2” on the Browning.  And for some
reason the Browning sweet spot locations were less conclusive using
this technique.
• Feeling the vibrations:
Hold the rifle vertical by the stock wrist with the barrel hanging
down.  Using a wooden mallet, piece of dowel rod, or a wooden
hammer handle; tap along the length of the barrel.  You should be
able to feel the vibrations.  The point of minimum vibrations is the
sweet spot.  This works but does not identify the sweet spot as
accurately as the following technique.
• Stethoscope method:
If you have a stethoscope handy or can borrow one, the following is a
version of the previous technique and is the easiest and by far best
method in my opinion.  Clamp the receiver in a vice and place the
stethoscope on a portion of the barrel above the forearm.  Using a
wooden mallet, piece of dowel rod, or a wooden hammer handle,
lightly tap along the length of the barrel.  You will easily hear the
pitch of the ringing change from a hollow sound to a lower frequency
or dull thud which identifies the sweet spot.  Leveling the barrel is
not necessary.  Here’s a good video on the technique in which the
presenter used a microphone taped to the barrel rather than a

A less accurate version of this technique is to place the stethoscope
against the end of the muzzle while lightly holding the rifle vertical
by the front sight or muzzle with the butt stock resting on your foot
or carpeted floor.  Lightly tap along the barrel to identify the sweet

If you’re unsure if the method you choose has correctly located the
sweet spot, try another.  They should approximately agree.  If not I’
ve found that the stethoscope method is the most accurate.  Once
the sweet spot is identified, mark it by placing a piece of tape at the
location and test it under actual match firing conditions with the help
of someone watching the barrel.  If the barrel still tends to jump a
bit off the cross sticks adjust the resting location slightly forward or
rearward of the tape location.  Once the location is firmly identified,
measure and note the distance to the muzzle since the tape will tend
to wear off or come off with cleaning solvents and oil.  Using masking
tape I’ve painted a ring around the barrel of my rifles using paint
impervious to cleaning solutions.

Wishing you great shooting,