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MEASURING THE GROOVE DIAMETERS OF ODD-GROOVE BORES By Wayne McLerran |

Posted 6/11/20

__Contents:__

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 http:

//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.

Notes:

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

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 http:

//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.

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.

Notes:

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.

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.

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.

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.

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.

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.

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.

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.

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:

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:

dimension C must be determined for each gauge. Following the instructions in

Powley’s

(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.

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)]2/[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 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.

Notes:

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.

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:

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.

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.

Notes:

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.

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,

Wayne

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,

Wayne