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How to
Slug A Bore
© 2010 and all subsequent years by h6x.net
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I'm the husband of the owner of this
website and I would be pleased to share with you some information that took me
years to figure out for myself.
There are two major reasons
why you will certainly need to slug the bore of your old military firearm...Safety & Accuracy! Since the bore diameter often varies quite a
bit with old surplus guns, you'll need to do this before you fire the first
round down the barrel (or purchase a box of bullets for reloading purposes). Once you know the exact "groove" diameter of
the bore, you can purchase the correct ammunition (or reload accordingly). Don't
be afraid to take a caliper or micrometer to your local gun shop so that you may
measure the bullet diameter before purchasing.
SAFETY: Basically, if you fire an oversized
bullet through a smaller bore, you could blow yourself and your gun to pieces.
Oversized bullets cause excessive chamber pressure!
Here's an example for
the Mosin-Nagant M1891: Most Russian, Soviet,
Chinese, Hungarian, Albanian, and Polish M91-type firearms tend to be at the
standard groove diameter of .311" (and larger...up to .316" in some
cases). Since most military surplus and commercial ammunition for this caliber tends to run
between .310" - .311", there isn't any real safety concern. However,
Finnish bores may run as small as .308" (and their standard groove diameter
is .310"). Therefore, if you fire some hot Russian commercial Barnaul
ammunition (@ .310") through your Finnish M/28, you're likely to have a bad
day. In most modern-day firearms, an oversized bullet wouldn't even chamber.
However, there's usually enough throat erosion (in the chamber) of these old
military surplus rifles that you'll never know there's a problem until after you
pull the trigger. YOU MUST ALWAYS CHECK YOUR WEAPON'S GROOVE DIAMETER AND THE
DIAMETER OF YOUR BULLET BEFORE FIRING!!! In any case, the first round you fire
should be with the gun strapped to an old tire with a 100' string attached to
the trigger.
Here's another example
pertaining to the Nagant M1895 revolver: The standard bore/groove diameter for
these guns are near .312" but WWII era firearms can be much
larger. Standard bullet diameters for military and commercial ammunition for
this caliber (7.62 Nagant) hovers around .3115". However, commercial
bullets (for reloading purposes) will either be at .312" or .314".
Basically, if you try to shoot .314" bullets through a .312" barrel,
you're asking for disaster.
ACCURACY: Usually, the best accuracy is achieved
when the bullet diameter is exactly the same as the bore's groove diameter (such
as a .311" bullet in a .311" groove diameter barrel). Sometimes, it's
possible to fire a bullet that is up to .001" larger than the groove
diameter but you're flirting with a dangerous situation here. When the groove
diameter becomes more than .002" larger than the bullet diameter, accuracy
starts to be a problem in most cases...anything over .005" and you may as
well start throwing rocks. To my knowledge, there are commercial hunting bullets
for 7.62x54R available in .308", .309", .311", .312", and .313"
diameters. This gives the reloader a fairly wide range when reloading for a
weird-sized groove diameter. Surplus ammunition for the M1891 seems to run between .309"
- .311" diameter while modern-day commercial ammunition seems to be at
.310" or .311". However, never make assumptions!
LANDS & GROOVES: What are we talking about
here? Inside your barrel is called the bore. Your bore is rifled with spiraling
lands & grooves which tends to spin & stabilize the bullet in flight. The
"Lands" are the high points...just like an island or land that arises
from an ocean. The "Grooves" are the low points. When you slug a
barrel (see below) this creates an opposite mold whereby the high points on the
slug are where the grooves of your rifling use to be. Due to excessive use (from
friction and hot gasses), the bore tends to become wider near the chamber and
tighter near the muzzle over time. Therefore, if you ever try to "sporterize"
an old military rifle by cutting back the barrel, your removing the best part of
the gun!
BASIC SLUGGING PROCEDURES: There are several
good tutorials on the Internet that will guide you step-by-step through this
process. However, the procedure is really quite simple so I will attempt to give
you a basic idea on how its done. First, you'll need to acquire a soft-lead ball
(such as used by black powder rifles). This ball should be just a tad bit larger
than your groove diameter. You will then forcefully drive this ball through your
barrel (being careful not to damage the sensitive crowning on the muzzle end).
This should be done using a hammer, wooden dowelled rod (for starting the slug,
especially if near the muzzle end), and another heavy-duty metal rod (at least
as long as the barrel length). At this point you simply measure the widest part
of the removed slug...this is your groove diameter. A digital micrometer or dial
caliper will work fine but someone using an older barrel micrometer can often
measure this diameter to within .00025" (¼ thousandth of an inch). In any
case, getting to within .001" is just fine. With this measurement, you are
now able to purchase the correct ammunition and/or bullets.
© 2010 and all subsequent years by h6x.net
All rights reserved.
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