A good way to slug a barrel is to push a lightweight lead bullet down it
from the breech end. That lead bullet should be a couple thousandths
of an inch bigger than the barrel's expected groove diameter so it will
swage down to the groove diameter. You can also use round lead balls,
but they're somewhat harder to measure with a micrometer. I've found the
best slugging cores have a bearing surface against the bore about equal
to the bore's diameter. Longer ones get a tad too hard to get started.
Shorter ones can be difficult to measure accurately.
Make slugging cores using a fired case from the barrel to be slugged.
After drilling a hole in the case head larger than the inside diameter of
the case mouth, put a bolt in through that hole up against the inside
of where the shoulder meets the neck. Then pour melted lead in the case
mouth and let it harden, and finally remove the bolt. What this does is
make a slugging core a couple thousandths bigger than the barrel's groove
diameter and hold that slug in alignment with the bore as its case is
chambered. Pushing a cleaning rod through the back of the case starts
that slugging core fairly straight into the rifling.
First, completely clean the barrel. Otherwise, fouling may make the lead
bullet hard to push through. If there's enough fouling, the diameter of
the lead bullet as it comes out of the muzzle may be a couple ten-thousandths
smaller than the actual groove diameter.
You'll need an accurate micrometer with a vernier that reads in
ten-thousandths of an inch. Having the micrometer calibrated and zeroed
against a gage block is a good idea if you want accurate measurements.
Use a solid, stainless steel cleaning rod without a tip, but put a brass,
flat-head screw in the rod's end. That screw head needs to be several
thousandths smaller than the bore diameter. With the flat-head screw in
the cleaning rod, the cleaning rod won't get off center on the slugging
core and possible gouge the rifling. Align the rod, then gently push
the lead bullet into the bore. Use even pressure as you push the lead
bullet down the bore. The initial resistance will be somewhat high, but
as the bullet gets past the start of the rifling, it pushes much easier.
An interesting thing about slugging rifle barrels is to notice the varying
resistance to pushing the bullet through it. When the resistance gets light,
than typically means either the bore or groove diameter is larger at that
point, or the bore is smoother at that point. With the resistance getting
heavier, the bore/groove gets smaller in diameter or the bore is rougher at
that point. It's difficult to tell what the real cause is.
As the bullet reaches the muzzle, the muzzle should be placed over a soft
rag to catch the bullet without deforming it. After the bullet falls out
the muzzle, measure it carefully with the micrometer. Measure across
opposite large diameter reliefed grooves between the relief lands engraved
into the lead bullet. For a 6.5mm barrel, it will probably be somewhere
between .262- and .265-in. in diameter. Pushing another lead bullet through,
then measuring it should repeat the first ones measurments.
Jacketed bullets tend to be most accurate when fired in a barrel whose
groove diameter is about half a thousandth of an inch smaller than the
bullet's diameter. Bullets as much as two thousandths of an inch larger
than groove diameter will shoot very accurate, as long as the bore and
groove dimensions are reasonably uniform.
