HOW TO Measure Odd Number Groove Diameter

[VMFn542bob]

HOW TO Measure Odd Number Groove Diameter
The following question was posted on the old "ASK EACH OTHER" forum by ccrider on 09/20/2009:
"Can some one explain how you measure a slug from a 5 grove barrel so as to determine bore size?"
It was answered by Bob S Posted - 09/20/2009:
"Wrap the slug with thin shim stock; mike the shim stock first, it may not be "as advertised". Wrap the slug tightly,
and mike the resulting wrapped slug very gently, so you don't compress the shim stock into a groove in the slug
(which is an imprint of a land in the bore). Subtract twice the thickness of the shim stock, and that is the groove diameter."

All I am doing here is demonstrating the technique put forward by Bob S. I think it is a good one.
I have only one such firearm, a 1873 45-70 Springfield Trapdoor rifle. It has 3 lands and 3 very shallow grooves.
Using the shim stock technique this is how I measured the groove diameter on my rifle.

1-All Purpose Machine Rest & Shooting Bench I use it on the range as a bench rest, a remote controlled trigger machine
rest and a gun cleaning rack located next to my gun safe and reloading bench at home. The 'Hyskore Dangerous Game Machine
Rest and Rifle Rest' is secured to the ambidextrous bench with two bolts for quick removal for transport and to free up the bench
for sandbag shooting.


2-Tools & Supplies Used
MAKING THE RAMRODs
I prepared two 7/16" (0.4375" diameter) hardwood dowels to use as ramrods to drive the slug through the bore, one 6" long to
start the slug and another to push it through to the breech.
The ends of the dowels were turned down a few thousands of an inch on a lathe to fit tightly inside fully sized 40S&W cartridges
with spent primers. One end of the dowel was secured by a Collete in the headstock and a 40S&W brass end cap was pressed
squarely on the other end using the tail stock feed. When the caps were tightly in place, the lathe was put into reverse and a cutting
bit, set above center, was used to crimp the mouth of the case onto the dowel.

PREPARING THE SLUG
Prepare or purchase one or more pure lead bullets of the caliber of the bore.
{Cast bullet molds often produce over sized bullet diameters that include the caliber (i.e. 30 caliber = .300 inches) plus twice the
depth of the groove and a little more for shrinkage.} If the prepared bullet slug diameter diameter is not at least as much as the
expected groove diameter to be slugged, place the bullet in a vice squarely between two flat hard surfaces and slowly squeeze
the bullet's length until the desired diameter has been achieved.
{I use a machinist's vice that has smooth jaw faces but any vice will work if the face of the jaws are covered with a smooth hard material.}
Lightly lubricate the bullet with any high viscosity grease, being sure to put grease in the grooves of the bullet.
{I used RIGG'S Grease.}


3-Slugging the bore
Place a pure lead bullet in muzzle and press by hand, nose first as far as possible.
With a mallet, tap the bullet into the bore until it is flush with the muzzle and the location of the bore is obvious.
{I used a rawhide mallet but a steel hammer would work if a piece of soft wood was held between the base of the bullet and the hammer.}
Using the mallet and the short ramrod drive the slug 4-5 inches into the bore and withdraw the short ramrod.
Use the long ramrod to continue driving the bullet through the breech and retrieve it for measurements.



4-Measuring the groove diameter
Wrap the brass shim around the slug, allowing the edge of the shim to hang over the base. I used a 1" x 5" piece of 0.002 and 0.005 half
hard brass shim stock and got the same measurements with both. Tempered shim stock would be better than half hard.
Anything thinner than 0.002 is likely to reduce the actual diameter measurement and anything thicker than 0.005 more may be too
difficult to pull tightly around the slug. Use a caliper to make several measurements at several different cross sectional areas.
Subtract twice the thickness of the shim from the measurement and that is the groove diameter. Care should be taken not to press the
shim into a groove on the slug.


Mission accomplished.

[joe wilson]

Very interesting, thanks. The pictures are great. I like that machine rest !

[HughUno]

option 2

buy anvil micrometer. measure slug.

[VMFn542bob]

buy anvil micrometer. measure slug.

How about giving us a tutorial. I have never used one or seen one.
I would think a 60 degree "V" Anvil Micrometer (the one you have illustrated) would be rather difficult for measuring a 5-groove slug.
Furthermore, the sharp point on the spindle (as shown) would be problematic for measuring to the surface of a soft lead 3-groove slug.
Do these micrometers come with conversion tables or is it built in to the display for 60 degrees?

[HughUno]

To measure five-groove barrel use a 108 degree anvil micrometer. a three-groove barrel needs a 60 degree anvil micrometer.

[VMFn542bob]

To measure five-groove barrel use a 108 degree anvil micrometer. a three-groove barrel needs a 60 degree anvil micrometer.

Thanks for furthering my education. With a little research I have learned that the V-Anvil (Tri-Flu) Micrometer's primary purpose is for precision measurements of cylinders and fluted cutting tools. I've never had any problem measuring cylinders with conventional micrometers.
The most common cutters I encounter are 2 and 4 flute end mills which can easily be measured with any conventional micrometer or caliper.
The 3 flute (60 degrees) V-Anvil micrometer appears to be plentiful but expensive.
I have found reference to a 5 flute V-Anvil micrometer but it is also scarcer than chicken teeth.
The V-Anvil micrometer seems to me to be a very expensive and questionable way to make a simple groove diameter measurement on a soft lead slug. If you were to use one you would not want to use the pointed one you show. That one seems more appropriate for measuring the depth of a thread.