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By Wayne McLerran
Posted 9/2/20

Slugging the bore
Spin-the-slug method
Shim-wrap method
Powler Gauge
Try-Rod Gauge
Pin/Plug Gauge
V-angle Micrometer

Most cast bullet shooters, especially reloaders, realize that matching
the bullet diameter to the bore groove diameter is critical to achieving
acceptable accuracy with a firearm.  The bullet diameters should
match or better yet slightly exceed (by 0.001” – 0.002”) the groove
diameter.  For a given caliber, groove diameters can vary by a few
thousands from manufacture to manufacture, especially so with 19th
and early 20th century firearms.  Although a machinist or gunsmith
may use specialized tools, the most common method to determine the
groove diameter starts with “slugging” the bore or making a
chamber/bore cast or impact impression.  If you’re unfamiliar with
either technique see my article at
com/Chamber_Casts_and_Impact_Impression.html.  Using a
micrometer or caliper, an accurate diameter measurement is easy
assuming the bore has an even number of lands and grooves.  Just
measure the slug or cast across opposite bore groove impressions.  The
problem arises when attempting to determine the groove diameter of
bores with odd numbers of lands and grooves.

Numerous old and modern firearms have odd-groove bores, 3 or 5-
groove being the most common.  The late 19th century Springfield
Trapdoor rifles had 3-groove rifling, of which many are currently
owned and continue to be shot.  Going back to the 18th century,
numerous flintlock and later caplock rifles had odd-groove bores which
were preferred so that a groove and land are on opposite sides of the
bore.  Modern Smith and Wesson revolvers have 5-groove bores as does
Ruger’s GP100 and SP101 revolvers.  No doubt there are others.  Even
groove bores may be more common now but some precision shooters
and barrel makers firmly believe that by placing the lands opposite of
the grooves the bullets are no longer squeezed on opposite sides
resulting in a more uniform and therefore potentially a more accurate
projectile.  If you’ve heard of “5R” (5-groove radial or ramped) rifling,
this technique is used plus the transition angle from the land to the
groove is either ramped or larger than 90° (radial rifling), resulting in a
bore that is also reportedly easier to clean.  By the way, if it’s not
obvious, the number of lands and grooves are always the same.  It’s
impossible to make a grooved bore with more or less lands than

Slugging the bore:
“Slugging the bore” typically consists of forcing a soft lead slug, slightly
larger than the groove diameter, down the bore from the breech or
chamber.  I generally use a .50 caliber soft lead ball for slugging .40
and .45 caliber rifles.  I’m also a black-powder muzzle-loader shooter
so pure lead balls are readily available.  But a soft lead cast bullet will
also work as long as the diameter is larger than the groove diameter of
the rifle.  If the bullet is not sufficiently large, squeeze it from end-to-
end in a vice to expand the diameter.  The ball or bullet may be driven
down the bore from the breech with a steel rod, which will not hurt
the bore assuming the end is flat without any burrs around the edges.  
For additional insurance tape the end of the rod.  A long wooden dowel
should also work, but if you value your cleaning rod refrain from using
1) From here on the term “slug” (see Figure 1 below) will be used
when referring to a slug, chamber cast or impact impression.
2) As noted in Figure 1, the ridges on the slug represent the grooves in
the bore.
3) When slugging a bore by forcing a soft lead slug completely through
the barrel, one situation that will result in a false reading is a
“choked” bore at the muzzle.  Pedersoli is known for slightly reducing
the bore diameter at the muzzle to increase accuracy.  If the bore is
choked it will become harder to push the slug as it nears the muzzle.  
In that case push a new slug from the breech part way down the bore
and push it back out of the breech.
Spin-the-slug method:
A rough measurement can be made, usually within 0.002”, possibly a
bit more, by slowly rotating the slug inside the jaws of the measuring
device so the jaws barely contact (just kisses) the edges of the groove
ridges on the slug.  See the purple line in Figure 2 below.  The
measurements will most likely be off (always less than the actual
diameter) a bit due to the groove corners being slightly rounded (not
truly at 90°).

This technique was used to measure an old CerroSafe cast from a 3-
groove Springfield Trapdoor rifle, the only odd-groove firearm I own.  
The results were compared to a very accurate measurement using a 3-
flute micrometer or tri-mic discussed later.  The spin-the-slug results
were 0.4633” vs. 0.4643” with the 3-flute tri-mic, an error of 0.001”
less than the tri-mic measurement.  But note, in this example the cast
was made from alloy a lot harder than a soft lead slug.  A slug made
from soft lead will likely result in a larger error due to further
rounding of the fragile groove edges by the jaws of the caliper or
Shim-wrap method:
Cut a thin (0.002” to 0.005” thick) strip of brass shim stock or a
section from a beer or soda can about 1/2" wide by 3” to 4” long.  
Wrap it around the slug.  Pinch it tight and hold it with pliers or
between thumb and finger of one hand.  Using calipers or a
micrometer, measure the diameter of the wrapped slug.  To minimize
errors due to flexing of the shim, measure the wrapped slug across the
edges of the groove ridges as illustrated in Figure 3.  Take several
measurements at different points and average the results.  Then
simply subtract 2 times the thickness of the shim to get the groove
diameter.  The bore or land diameter can be determined by measuring
the depth of one groove at one of the groove-to-land transition steps
and subtract twice the amount from the groove diameter.
Note –Shim material thicker than 0.005” may be too difficult to wrap
tightly around the slug, resulting in too large of a diameter reading.

To give you some idea of the accuracy with this technique, 0.002” to
0.005” shim stock was used to measure the CerroSafe cast mentioned
earlier from a 3-groove Springfield Trapdoor rifle.  As long as the slug
was measured across the edges of the groove ridges the thickness of
the shim material did not matter.  After subtracting the shim
thickness, all measurements generally agreed to within an average of
0.001” less than the actual diameter as measured with the very
accurate 3-flute tri-mic.  Due to flexing of the shim in the grooves of
the slug, if the slug is not measured across the edges of the groove
ridges the error will increase.  In that case the thinner the shim the
larger the error.  By the way, to ensure your technique is correct,
measure a drill shank of similar diameter with this method.  After
subtracting the thickness of the shim the result should agreed with the
diameter of the drill shank, if not then you’re likely not squeezing the
shim sufficiently tight.
Powley Gauge:
Homer S. Powley, considered by some to be one of the greatest
ballisticians of his time, is well known for his slide-rule-type “Powley
Computer for Handloaders” developed in the early 1960’s.  In a short
article titled “Measuring 5-Groove Barrels”, published in the 6th
(1972) edition of Handloader’s Digest (page 51), Homer described and
illustrated a V-angle gauge configured to easily measure the groove
diameter of 5-groove bores.  See Figure 4.  The same or similar
articles may have been published in other editions of the Handloader’s
Digest or Lyman’s Cast Bullet Handbook.  Since then the gauge is
referred to as the “Powley Gauge”.  The included “V”-angle of the
gauge is the same as the jaw angle in a 5-flute V-angle micrometer.  
So the basis of the technique is not new but Homer’s simple gauge can
be made by a machinist at almost no cost for the material.
Notes: As illustrated in Figure 4, when using the gauge the equation is:
diameter (D) is equal to 0.8944 times B or D=0.8944•B.  For this
method and discussion, 0.8944 will be referred to as the Powley
Number (PN).  How the PN was derived and the correct PN to use with
gauges for 3 and 7-groove slugs will be covered later in this article.
Mike Deland, a very experienced machinist and frequent contributor
to several firearm forums, made and has used a Powley Gauge.  He
provided photos of his gauge below (Figure 5).  Although the gauge is
ideally designed for 5-groove bores, Mike commented that it will also
measure the majority of slugs from 3-grove and 7-groove bores.  Note
that he stamped the Powley equation, dimension C and the PN on the
face of the gauge.
To verify Mike’s comments I worked up separate illustrations (see
figure 6) for 3, 5 and 7-groove slugs using a 5-groove Powley Gauge
which has an included angle of 108°.  As the illustrations confirm, the
gauge, with limitations, will also measure 3 and 7-groove slugs.  The
limitation is that some portion of the groove contact points must be
tangent to the angular sides the gauge for an accurate measurement,
and it cannot be the ends of the grooves which may be slightly
rounded.  Due to the length of the slugs and gauge and also the
groove pitch or twist rate of the bore, there is a range of unequal 3
and 7-groove groove widths vs. land widths the gauge will measure.  
But if the bore grooves are significantly narrower than the lands the
gauge will not work.  In that case a Powley-type gauge with different
included V-angles will be necessary, more on this later.
Deriving the Powley Number (PN):
Should you decide to make one or more Powley-type gauges to
measure 3, 5 and 7-groove slugs, the Powley Number (PN) will be
different for each gauge.  So, let’s use a couple of equations and the
5-groove Powley Gauge (see Figure 7) as the basis to confirm the PN
that Homer used in Figure 4.  The same equations will be used to
determine the PN for the other gauges.
If D is the groove diameter then D/2 is the groove radius as illustrated
in Figure 7 along with the angle α used in the equation below.  
Without going into how it was derived, the 1st equation is used to
determine the groove diameter without using the PN.

A=C+(D/2)(1+1/cosine α)
Rearranged as D/2=(A-C)/(1+1/cosine α)
Rearranged again as D=2(A-C)/(1+1/cosine α)
Since B=A-C in Figure 7 the final equation is D=2(B)/(1+1/cosine α)

As illustrated in Figure 7, angle α is 36° and B=A-C.  So let’s assign the
value of 0.7” to A and 0.2” to C for this example.  Therefore B=0.5”.  
And using an online calculator, the cosine of 36° is 0.8092.  So the
equation becomes D=2(0.5”)/(1+1/0.8092).  Therefore the groove
diameter D=1”/(1+1.2358) = 1”/2.2358 or 0.44726”.

The 2nd equation is the Powley equation as illustrated in Figure 4 as
D=0.8944•B or:
D=PN(B), rearranged as PN=D/B.
Therefore, using the groove diameter derived above, PN=0.44726”/0.
5” or 0.8945.  So Homer’s PN value (0.8944) is off by only 0.0001.

I noted earlier that a Powley-type gauge with different included V-
angles may be necessary for measuring 3 and 7-groove slugs with
unequal width grooves.  In that case the ideal included angles would
be 60° for 3-groove and 128.57° (128°, 34’, 17”) for 7-groove
gauges.  A V-angle gauge of 60° has an α of 60°.  A V-angle gauge of
128.57° has an α of 25.71°.  Using the same equations above results
in the following PN values:
3-groove gauge:
α = 60°, cosine of 60° = 0.5, therefore the PN = 0.6667
7-groove gauge:
α = 25.71°, cosine of 25.71° = 0.9010, therefore the PN = 0.9479

Using a test rod and PN to derive dimension C:
Finally, to use the Powley formula for any of the three gauges
constructed, dimension C must be determined for each gauge.  
Following the instructions in Powley’s
Handloader’s Digest article
(Figure 4), using a carefully measured rod (drill rod, drill shank, etc.),
dimension B was first determined by multiplying the rod diameter by
1.1180.  What the article does not mention is 1.1180 is actually the
reciprocal of the PN value or 1/PN, or in this case 1/0.8944.  It’s
derived using the Powley equation rearranged as B=D/PN or B=D
(1/PN).  Therefore dimension B is easy to calculate using the test rod
diameter for D and the appropriate PN calculated earlier for your
gauge.  Now that you know dimension B, carefully measure dimension
A with the rod in place and use the equation C=A-B to calculate
dimension C for the gauge.
Note – Dimension B will be different for every rod or slug being
measured with a specific gauge but dimension C will not change.

Tri-Rod Gauge:
The Tri-Rod Gauge is similar to the Powley Gauge and is easy to
construct without a milling machine.  The gauge is carefully made of 3
rods or tubes of equal and uniform outside diameter (OD).  If precise
measurements are made with calipers the results are as accurate as
using the Powley Gauge, certainly within 0.001”.  In lieu of rods,
metal tubing can be used.  I considered some thick wall PVC tubing but
the OD was not consistent when rotating the tubing while measuring it
with calipers.

The rods and tubes are strapped, glued, silver-soldered or welded
tightly together, being sure that all three touch evenly along the
length of the gauge.  See figures 8 and 9.  I found using super glue was
the fastest and easiest method to quickly construct the gauge.  
Although super glue may be durable I went one step further and filled
the center triangular portion where the tubes meet with epoxy to
ensure it would not come apart.

The slug to be measured is placed in the channel where two of the
rods or tubes meet.  With one bore groove centered at the top, two of
the grooves must evenly contact the two rods or tubes.  Since the
gauge sides are not flat as with the Powley Gauge, one gauge may not
work for measuring 3, 5, and 7-groove slugs even if the grooves and
lands are the same widths.

You may be a machinist with a lathe, but even so by far the easiest
method is to head to your local hardware store and purchase copper
tubing couplings.  I bought three each of two sizes, 1/2” and 3/4" for
less than $5 total from Home Depot.  The dimensions do not specify
the diameter or length of the couplings but the size of the copper
tubing there’re made for.  Even a gauge made from the small
couplings for 1/2" tubing was sufficiently long to measure any length
slug.  The couplings were uniform in diameter and no cutting was
required.  But check closely for any damaged prior to purchase and,
using sandpaper, remove the rough outer edges around the ends.  
Although I did make another with smaller diameter aluminum tubing,
the two gauges made from copper couplings (see Figure 8) will
measure a slug from any odd-groove barrel.  When making a gauge a
good rule-of-thumb is to use rods or tubes with ODs about 1.5X to 2X
the diameter of the slug.  Note that pictured with the gauges is the
Trapdoor rifle 3-groove CerroSafe slug used with this technique and
the others to verify the accuracy of the measurements.
As illustrated in Figure 9 the variables are H (height of the gauge plus
the slug) and R (radius or 1/2 the diameter of the rod or 1/2 the OD
of the tubing used).  D is the resulting slug groove diameter and 1.732
is the square root of 3.  Following is the equation without going into
how it was derived:

D = [H-R-R(1.732)] X [H-R-R(1.732)] / [H-R(1.732)]

Using the middle size copper fitting gauge depicted in Figure 8, the
Trapdoor rifle CerroSafe cast was measured and the results compared
to the groove diameter determined with the 3-flute tri-mic.  Values
used were: H=2.0620”; R=0.4778”.  The result was a groove diameter
(D) of 0.4638” vs. the tri-mic value of 0.4643”, an error of only

Since the bottom of the gauge is rounded, not flat as with the Powley
Gauge, it’s a little “trickier” to measure the height of the gauge and
slug.  When determining “H” make certain the measurement is made
from the bottom center of the gauge to the top center of the slug by
slightly rotating the gauge and slug together back and forth in the
jaws of the caliper to obtain the largest value.

Use a drill shank or drill rod with a diameter you’ve confirmed to
verify the accuracy using all three channels.  If the rods or tubing
used for the gauge are uniform and were attached together correctly
the measurement of the total height (H) of the gauge and test rod
should not vary by more than 0.001” between the three channels.  
After checking all three, if they do not agree, run the calculations and
use the channel that closely agrees with the test rod diameter.

Pin/plug-gauge method:
If you have a good set of pin and/or plug gauges (see note below),
most shooters do not unless you’re a machinist; they can be used to
quickly determine the bore (land) diameter and a close approximation
of the groove diameter.  Insert the largest one that will fit snug in the
muzzle which will be the bore diameter.  Then add 0.007” to 0.008”
(twice the typical groove height of cast bullet rifle bores) to obtain a
close approximation (usually within 0.002”) of the groove diameter.  A
more accurate method to determine the groove height is to slug the
bore and measure the height of the groove-to-ridge transition step
with the rear end of calipers, then add twice the amount to the bore
1) The above method will not work with a “choked” bore at the
muzzle.  See the earlier note on Pedersoli rifles.
2) Pin gauges and plug gauges are essentially the same but may be
differentiated by size.  I.e., a pin gauge is a small plug gauge and a
plug gauge is a large pin gauge.

Using V-anvil micrometers (aka tri-mics):
The first time I needed to measure an odd numbered groove bore was
many years ago after acquiring a model 1884 Springfield Trapdoor
carbine.  Springfield Trapdoor rifles and carbines have 3 lands and
grooves and are notorious for having a wide range of groove
diameters.  Since a Trapdoor rifle does not offer straight-line access
to the chamber it’s almost impossible to drive even a pure lead slug
down the bore from the chamber.  One option is to drive a slug down
from the muzzle but the muzzle tends to shear off the larger outer
diameter portion of the slug rather than forcing the lead to
completely fill the grooves, resulting in a less accurate slug
formation.  Therefore a chamber/bore cast or impact impression are
the only good options I’m aware of.  The CerroSafe slug pictured in
Figure 8 was cut off the end of the chamber/bore casting.  To
measure the slug I purchased a 3-flute V-angle micrometer.

Using a 3-flute V-angle micrometer to measure a 3-groove casting or
slug is by far the easiest and most accurate method, but V-angle
micrometers are expensive, generally in the $250 to $800 range
depending on manufacturer and model.  I was fortunate to find a nice
used one on eBay at the time for $60.  New 3-flute V-anvil
micrometers are plentiful.  5-flute models are available and generally
cost more than 3-flute.  7-flute versions can be quite expensive,
especially the electronic models, hard to find and most are based on
the metric system rather than the US/Imperial inch measuring units.

Note - as indicated earlier, the 5-groove Powley Gauge can be used to
measure some 3 and 7-groove slugs with unequal width grooves and
lands, but due to the narrow jaws of tri-mics it’s unlikely that 3 and 7-
groove slugs can be measured with a 5-flute V-angle micrometer.
So there you have it, several techniques to measure the groove
diameter of odd-numbered bores, some costing nothing to an
expensive solution.  I doubt anyone reading this will be rushing to
purchasing one or more tri-mics and very few have a set of pin or
plug gauges.  Therefore if you’re a machinist or have a machinist
friend consider a Powley Gauge.  For most others the Tri-Rod Gauge
is the best solution followed by the shim-wrap method, which is fast
but expect the results to be 0.001” or so less than the actual
diameter with careful measurements.  The spin-the-slug technique is
certainly the fastest but can result is large errors when measuring
soft lead slugs.

I thought I’d close with some comments for those of you that have a
Springfield Trapdoor rifle since I mentioned it several times and used
a CerroSafe cast slug from it to verify the accuracy of three of the
methods discussed.  As noted the cast was old and CerroSafe enlarges
over time up to a certain point then stops.  Using the correct
expansion factor means the 0.4643” diameter cast represents an
actual bore groove diameter of 0.4619”.  The CerroSafe time related
expansion factors can be found in the article link referenced in the
opening paragraph of this article.

Wishing you great shooting,