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

Posted 9/2/20

__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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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:

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:

constructed, dimension C must be determined for each gauge.

Following the instructions in Powley’s

(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.

measured with a specific gauge but dimension C will not change.

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

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.

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.

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.

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,

Wayne

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