From: bartbob@aol.com (Bartbob)
Newsgroups: rec.guns
Subject: Re: Bottleneck Cases - Seating and headspace
Date: 11 Jan 1996 09:34:11 -0500

Your questions about reloading the .308 Winchester are good.

Headspace for the .308 Winchester, as well as other rimless,
bottleneck cases, is measured from the closed bolt's face to
a datum point on the shoulder.  This datum point for the
.308 Win. is .400-inch.  Minimum .308 Win. chamber headspace
is 1.630-inch.  Maximum is about 1.650-inch.  New cases
typically measure about 1.628-inch from case head to the
shoulder's datum point so they will easily fit in minimum-
headspace chambers, yet not stretch in firing enough to
cause head-separation problems when fired in a maximum-
headspace chamber.  New cases typically end up with a fired-
case headspace dimension of about .001-inch shorter than the
chamber's actual headspace.

Headspace is measured with steel gages precision ground to
specific dimensions.  They are put in the rifle's chamber,
then the bolt is gently closed.  If the chamber is too short
and a minimum gage is used, the bolt won't fully close; the 
chamber must be lengthened with a reamer to fix the problem
as new cases may not allow the bolt to be closed.  If the
chamber headspace is between the minimum and maximum limits,
then the minimum gage will let the bolt easily close.  If a
maximum gage is put in the chamber, the bolt should not be
able to close which verifies the chamber headspace is within
manufacturing limits for safe use with new cases.  But if
the bolt closes on a maximum gage, chamber headspace is too
long and new cases may rupture if fired in such a chamber.
Minimum gages are called "go" gages, maximum gages are
called "no-go" gages based on the idea that a the bolt 
should "go" closed on a go-gage and "no-go" closed on a
no-go gage.

Bottleneck sized- or new-case headspace can be measured with
a gage such as the RCBS Precision Mic.  A case is put head-
down in the gage, then a micrometer thimble is screwed on
and tightened until it stops against the shoulder.  The gage
is read in thousandths of an inch.  Loaded rounds can also
be measured with such gages.

Here's what was learned back in the 1960s about sizing
cases.  It applies to the .308 as well as other rounds of
similar size.  Tests were conducted with super-accurate
.308 Win. match rifles.  Chamber headspace was 1.630-inch,
or minimum.  Rifles were held in an unrestricted machine
rest.  An unrestricted machine rest clamps the rifle's
forend and butt much like it would be held by someone.  On
firing, it moves with the same resistance for each shot.
It slides on three steel rods riding in V-blocks and moves
about 3 inches when fired.  The upper cradle is moved back
forward against a stop after each shot and repositions the
rifle exactly the same for each shot.  Such machine rests
eliminates all human variables in holding and are commonly
used by bullet making companies and military arsenals for
accuracy testing.  Sometimes only a barreled action is
clamped in the rest; pictures of such machine rests are
shown in Sierra's reloading manual.  The US Olympic Shooting
Team uses similar ones for testing .22 rimfire match rifles
and ammunition as they know humans can't test them as well
as the rest can.

Such tests are much, much better than shooting a highpowered
rifle from a bench because of one important, but little
known reason.  The more recoil a rifle has, the more very,
very slight changes in how it's held effect how it moves as
the bullet goes down the barrel; very small changes cause
the barrel axis to be different for each shot as the bullet
leaves.  Rifles used in 100 to 300 yard benchrest matches
shoot mild cartridges with small powder charges and light
bullets which are shot in virtually free-recoil conditions
unimpeded by differences in holding which is near machine
rest conditions.  When held firmly like rifles shooting
larger, more powerful cartridges have to be, they typically
shoot much larger groups than the sub 1/4th-MOA ones they
are famous for.  Rifles tested in machine rests typically
have groups with smaller spreads between largest and
smallest ones than when conventionally benchrested.  What
this means is that most highpowered rifles actually shoot
better than conventional benchrest techniques show.

The .308 rifles fired in a machine rest would shoot about
1/2-MOA test groups with cases sized correctly for best
accuracy.  Note that twenty or more shots were fired for
each group. When hand-held and shouldered from a typical
benchrest with the rifle's forend and stock toe on sand
bags, the groups would be in the 1 MOA range.  Here's a
summary of what was learned from these tests.

When a full-length sizing die was set in the reloading press
as instructions said, sized cases had a head-to-shoulder
headspace typically shorter than a new case.  When fired,
these cases produced test groups about 1.5-MOA.  Note that
the die was set in the press such that its bottom just
touched the shellholder as the ram was at the top of its
movement.  This caused the case shoulder to be set back so
the sized-case headspace dimension was typically shorter
than that of a new case.  Such instructions supplied with
reloading presses and dies are required for two reasons
regarding chambers with minimum headspace dimensions. First,
the great variety of chamber sizes for a given cartridge
vary quite a bit and the manufacturer wants to be sure sized
cases will chamber properly.  Second, the amount of case
lube applied causes sized-case headspace to vary several
thousandths of an inch; lightly lubed cases will have a
longer headspace and the manufacturer wants to be sure cases
so sized will fit in minimum headspace chambers.  Cases so
sized could be reloaded about 10 times before their head
separation became imminent.

Fired cases partially sized in a full-length sizing die
with about three-fourths of the neck sized produced test
groups about 1.5-MOA groups.  Cases so sized had their
body diameter's reduced a couple of thousandths of an inch
which moved the shoulder forward several thousandths of an
inch.  These partially-sized cases were actually longer in
head-to-shoulder datum point dimension than the chamber's
headspace dimension.  A slight binding was noted as the
rounds were chambered.  Case life of these cases was about
20 to 30 reloads.

Fired cases that were neck-only sized in dies that didn't
have their body diameters reduced but their necks were sized
down would shoot groups about 1-MOA groups.  Subsequent
firings of the same case resulted in its shoulder moving
forward enough to cause very slight binding as the bolt was
closed and groups opened up about 1/4th MOA with such cases.
About 30 to 40 reloads per case was possible.

New, never fired cases produced test groups about 3/4ths-MOA
even with all their irregularities from manufacturing.

The smallest test groups came from cases full-length sized
such as the shoulder was not set back from it fired
position.  Sized-case headspace was the same as, or not more
than .001-inch shorter than fired-case headspace.  Test
groups were about 1/2 MOA.  And cases could be reloaded
20 to 30 times.

This well explains why most folks get better accuracy with
partial-sized cases using full-length sizing dies than by
following press/die instructions for full-length sizing.  It
also explains why they get better accuracy with neck-only
sized cases compared to conventional full-length sizing with
the die set according to supplied instructions.

It should be mentioned that the machine-rest tests were made
at 600 yards.  Had the tests been done at 100 yards, the
smallest groups would have been about 2/10ths-MOA and the
largest ones about 3/4ths-MOA.  In one test, 40 consecutive
shots were fired into just under 2 inches at 600 yards.  To
do that well, the group at 100 yards would have to be about
1/10th MOA.

By using a sized-case headspace gage like the RCBS Precision
Mic, you can set a full-length sizing die in a press such
that it sizes the cases just enough to set the shoulder back
no more than .001-inch.  You'll probably get best accuracy
this way as well as excellent case life.