Most gun dimensions can be measured
with a 6" caliper, a 1" micrometer caliper and a steel tape measure.
The 6" caliper is either a vernier or dial or digital (electronic) caliper
that reads to .001", a thousandth of an inch. I find a dial caliper easier
to read than a vernier caliper and less expensive than a digital caliper.
There are plastic vernier calipers available, I have used them and don't
consider them of much value.
The 1" micrometer caliper measure to .0001", a tenth of a thousandth of an
inch. These come in mechanical or digital varieties, I prefer the
Steel tape measures measure to 1/16", one sixteenth of an inch. Learning
to use these instruments is not difficult, but it helps to have a person
show you the tricks.
Most gun dimensions can be measured with these instruments, with one
exception. Slugging the barrels of guns with odd numbers of lands and
grooves gives you slugs that CANNOT be accurately measured with these
instruments. More complex and involved equipment is necessary. Any
competent machinist can measure these slugs.
(Bill McGraw contends that it is possible to measure the groove diameter
of odd-number-of-groove barrels with a micrometer. I haven't been able to
measure five groove S&W barrel slugs or Trap Door Springfield 3 groove
slugs with a micrometer. Bill suggests that another way to measure the
slugs is to try them in a sizing die; if sized any, the slug groove
dimensions are larger than the die; if not sized, the die is larger.)
O. H. McKagen can measure these slugs free of charge for CBA members. Mail
your slugs in a padded envelope, each marked as to what part of the barrel
they were formed in, and include a SASE for a return reply. His address is
9229 Arlington Blvd., Apt. 551, Fairfax, VA 22031 (He offers this service
in The Fouling Shot, (Journal of the Cast Bullet Association); look for
any address changes.
HOW TO SLUG A
Slugging a barrel is the operation of driving a soft lead slug through a
bore to determine the bore and groove dimensions This enables us to
determine the proper cast bullet nose and throat diameter'(s) so that the
best accuracy may be obtained in that particular firearm. Some cast
bullets have two diameters, some have just one diameter, and some are
Slug rifle barrels at both ends since some guns have barrels that are
larger at one end than at the other. This is of some importance. As
a rifle is fired and cleaned, the chamber end wears bigger, faster, than
the rest of the barrel. This happens sometimes in surprisingly few shots.
If the object of slugging the barrel is to select bullet diameter, it is
the chamber-end diameter that is of most importance. Pushing a lead slug
through a barrel results in a slug of the smallest diameter of the
barrel that that slug passes through.
There are reports that some rifle barrels are constricted, are smaller,
where a band for a front sight or sling swivel has been swaged onto the
barrel. There are also reports that a constriction can sometimes be found
in a barrel where the printing is pressed into the metal. Ruger single
shot rifles are frequently mentioned, where the sling swivel band
installation may swage the barrel down several thousandths of an inch.
There are also frequent reports that revolver barrels are squeezed
undersize at the rear, where the barrel screws into the frame of the
The most important thing about slugging a barrel is not to damage the
barrel, especially the crown-the forward end of the barrel. Be careful!!
Get some soft bullets or muzzle-loader balls that are bigger than the bore
of the gun. If you can't get bullets bigger than your bore, take a lead
bullet of the same caliber as your bore and squeeze it in a vice or tap it
with a hammer to make it shorter and bigger in diameter.
To slug the muzzle end of the barrel, tap a bullet into the muzzle with a
plastic mallet and a brass drift or wooden dowel. Do not use a steel
hammer, even for the first few blows. No, donít even use that little tiny
toy steel hammer. (If you use a steel hammer you will probably have to
become familiar with barrel crowning, which is covered elsewhere.) Then
carefully tap the slug out of the barrel with a cleaning rod and screw.
(See the Appendix about the cleaning rod and screws.) Measure the slug to
get the bore and groove diameters.
As long as there
are either a lot of grooves or an even number of grooves, measurement is
easy. When there are a small odd number of grooves, measurement is more
difficult. The Trap Door Springfield has a hard-to-measure barrel with its
three grooves, as is a Smith and Wesson revolver barrel with five lands
Barrels with an odd number of grooves take special equipment to get
accurate measurements because of their unique geometry. The best approach
here is to become friends with a machinist, and complain to him until he
measures the slug. Constant moaning and complaining to my machinist
friends, amateur and professional, has gotten me a lot of work done and an
To slug the breech end of the barrel, drop an oversize bullet in the
chamber, put in a plugged case, put a brass drift or wooden dowel on the
primer end of the plugged case and tap the other end of the drift or dowel
with a plastic mallet until the bullet is up into the rifling. (Use a
rawhide or plastic mallet, not a steel hammer!) Whacking anywhere on the
action end of your rifle with a steel hammer will cause you to say bad
words and be a generally mean person for several weeks.) Then tap the end
of the slug out with a cleaning rod and screw, and measure the bore and
groove diameters. (See: How to
slug a rifle throat)
Be aware that new revolver bores often have a constriction in the area
where the barrel screws into the frame so you may want to note this
To slug a revolver barrel, tap an oversized slug completely through the
barrel to get a slug with the smallest diameter, possibly/probably
where the barrel is screwed into the frame and
down. Slug the cylinder chamber throats as well.
To find the barrel dimensions after the constriction, put a drift into the
barrel and down into the cylinder area. If, for example, the barrel is six
inches and the cylinder is two inches, use a five inch drift. Now put
quarter inch or so strip of steel, aluminum or brass between the drift and
recoil shield; this will allow you to drive the drift and slug out. Drive
a slug into the muzzle end of the barrel until it just touches the
drift-with the five inch drift mentioned above, the bullet will be just
shy of three inches into the barrel. Using the strip of steel, aluminum or
brass and the drift, knock the slug back out the muzzle end of the barrel.
This slug will have the barrel dimensions, bore and groove, of the barrel
where it is not squeezed down by the frame.
Slugging a barrel takes a little practice, be ready to make several slugs
of each end of the barrel. Remember; donít damage the barrel, and WRITE
How To Slug
a Rifle THROAT
Use a cartridge case that fits in the rifle. Fill the case with lead,
halfway up the neck. In this case, it took two bullets melted in the case
with my trusty Bernz-O-Matic.
Now put a soft bullet in the cartridge case, and put the case with bullet
in the rifle.
Use a piece of steel rod. In this case,
for a 30-caliber rifle I used a quarter-inch steel rod three feet long,
from the hardware store. File the business end so there are no sharp
edges. Put masking tape on the end, and every five inches or so. The tape
shims the rod away from the bore of the barrel.
Put the rod in the barrel until it touches the bullet in the cartridge
case. Then put masking tap on the rod at the muzzle, so that the rod
doesn't touch the bore at the
Here's the end of the rod, with tape.
Tap the end of
the rod with a steel hammer until you
feel it bottom - the rod will bounce. Open the rifle and gently tap out
the cartridge case and slug.
Here's the throat slug. In this
case it showed me that the chamber was cut
for a cartridge case 2.115"
long rather than the 2.039" standard, and that
there is no throat to speak of, merely a bevel from the end of the case to
the rifling origin.
From Ken Mollohan:
'How Too' on slugging throats. I always did it slightly differently: Drop
a steel rod in the case, and mark it at the mouth, so's I could cut it off
flush with the mouth of the case. Drop a couple of buckshot in the throat,
chamber the case (with rod) and pound them out with a rod much like you
described. Gets to the same place, but with a bit less trouble; but I like
yours because it gives you an OAL to the origin of the rifling.
How To Make
Sulfur Barrel And Chamber Castings
and most accurate method of making an impression to measure the throat
and/or the barrel is to slug the throat or barrel as described above.
Castings of the chamber are best made with sulfur or
An alternative to slugging the barrel is to make sulfur castings of the
breech and muzzle ends of the barrel. It is easy to make a casting of the
chamber while making a casting of the barrel. I hadn't made a sulfur
chamber cast for a few years, so to check my memory I went through the
I ordered 12 ounces of "Sublimated Sulfur" from the pharmacy, for $6.83,
in 2003. It was delivered the next day.
It took three tries to make an acceptable casting, partly because the gun
I chose is a Martini with limited space to work in. Eventually I had a
casting of the chamber and about 1" of the bore that was free of voids.
Sulfur castings are about .002" smaller in diameter than the lead impact
slugs of the bore or chamber, in the diameter range of .3"-.4".
It was suggested
that I try adding one part powdered graphite to two parts sulfur to
stabilize the casting. I did and made another casting. With the graphite
added the sulfur casting is about .001" smaller in diameter than the lead
The lead impact slug has the largest dimensions of any cast or slug I've
made of a particular gun, and I suspect that it gives the most accurate
dimensions of the throat-bore.
Here's how to
make a sulfur casting:
Sulfur will not melt in a double boiler; a higher heat is needed. A tin
can or an aluminum measuring cup over a low heat, watched carefully, works
Oil the barrel very lightly, and then plug it with a tight cleaning patch
where the casting is to end. A casting with one inch of barrel is about
For a muzzle end casting,
put masking tape around the end of and beyond the end of the barrel.Make a
sort of open-ended funnel with the tape at the end of the barrel, which
will hold some sulfur and keep it from spilling. Stand the rifle muzzle
up. Warm the sulfur over a very low heat. It doesn't take much heat, if
you cough, the sulfur's too hot!! Warm the end of the barrel slightly with
a Bernz-O-Matic torch until it is hot to the touch. Not too hot! Make sure
that the bore is plugged! (I forgot on my second attempt.) Pour the melted
sulfur into the funnel. Wait until the barrel is cool and sulfur is
hard-this may take half an hour-and then knock the casting out of the
barrel carefully from the breech end with the brass screw in the cleaning
rod. (See the Appendix about the brass screws in the cleaning rod.)
Measure the bore and groove diameters, and write them down.
Making A Chamber And
Remove the bolt or breech block and extractor from the rifle, use masking
tape and/or modeling clay to block up where an over-enthusiastic pouring
of sulfur would drool out, and proceed as explained above. Use a funnel,
or make a funnel from a manila file folder, if needed. When the barrel is
cool and the sulfur is hard, drive out the casting with the brass screw in
the cleaning rod. Make a sketch of the chamber with all dimensions and
measure the bore and groove of the barrel. Write it down.
Alternative casting materials such as "CerroSafe" (a low temperature
melting point metal alloy) are available from Brownells. Some shooters
have reported that Cerrosafe castings change dimension in time, making the
castings inaccurate. I have never used this material, so you're on your
HOW TO MAKE
CERROSAFE BARREL AND CHAMBER CASTINGS
(The easiest and
most accurate method of making an impression to measure the throat and/or
the barrel is to slug the throat or barrel as described above. Castings of
the chamber are best made with sulfur or cerrosafe. joe b.)
Cerrosafe is pretty easy to use, if a little care is taken with it. First
step is to clean the chamber and bore, dry patching when done. Take a dry
patch and cut it to size so that it can be folded in half and rolled into
a bore-fitting cylinder. Shove the patch into the bore from the muzzle,
with the folded edge going in first. Use your cleaning rod to run it to
within 1/2" of the chamber's throat, so that you can get a cast of the
bore at the breech.
Next, strip the barreled action from the stock and clamp the action
upright in a padded bench vise, so that you have access to the chamber.
I've never needed to warm up the
barrel, but if you do, don't go over 100 Fahrenheit.
Take a clean round can, I use a tuna can, and bend a pouring spout on one
side. I like to have the spout extend about 5/16" from the can body. Put
in your Cerrosafe, and clamp a small Vise Grip pliers on the side of the
can, 90 degrees from the pouring spout. Using an electric heat gun, heat
the can from the bottom until the Cerrosafe is just molten. Quickly pour
the casting metal into the chamber, stopping when it's just past level
with the chamber mouth. When you see it solidify--just like the sprue when
casting bullets--wait one minute.
Using a cleaning rod with a jag, push the casting onto a couple of shop
towels. If you wait too long, you'll have to heat the barrel and melt it
out! Two hours after the casting has cooled to room temperature, you can
get an honest measurement. Cerrosafe shrinks on initial cooling, then
expands as it ages.