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By Wayne McLerran
Posted 6/11/20

Slugging the bore
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
//www.texas-mac.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 grooves.

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 it.
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 micrometer.
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
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:


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 0.0005”

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 diameter.
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

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,