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From: megatek!hollen@UCSD.EDU (Dion Hollenbeck)
Newsgroups: rec.guns
Subject: Re: Rifle Chamber Measurements (Part 1)
Message-ID: <35273@mimsy.umd.edu>
Date: 5 Jun 91 19:02:34 GMT
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In article <35247@mimsy.umd.edu> hes@ccvr1.cc.ncsu.edu (Henry E. Schaffer) writes:
# Note that this is clearly in the acceptable area, but also notice
# how little difference there is.  (The danger is that if the case
# neck is long enough to jam into the end of the chamber, it will
# put pressure on the bullet and hold it in after ignition.  This
# will cause an increase in pressure within the chamber, and can
# lead to such undesirable results as blowing up the gun!)

Henry is completely accurate in this respect, with one exception.
Benchrest shooters (myself included) when fire-forming new brass
will typically seat the bullet out so far that it is inserted
into the rifling about .0005".  The bullet in the rifling and
the base in the boltface squeeze the cartridge in much the same
manner as the the head and tailstock in a lathe and cause the
the cartridge to be centered in the chamber as much as is possibly.

When the round is fired, the bullet indeed does hold in the pressure
for a very long time, and this is exactly what the benchrest shooter
is looking for.  While the pressure is building up, the brass is heating
and expanding and by the time the bullet has begun to leave the case,
the brass has expanded to EXACTLY fill the chamber.  As the brass cools,
it will shrink a few ten-thousandths and can be extracted easily without
binding.

Here is the very dangerous part.  We are trying to create a high-
pressure situation, but under control.  Loads are typically 
powdered at the extreme low end of the range of the particular
recommended range for that powder, end even lower than that.  
An experienced benchrest reloader will very slowly work the powder
up in extremely small increments from that until signs of over 
pressure are detected (primers flattened or backing out, etc.),
and then back down just a little, getting the maximum expansion
potential without the danger.

The big difference here is that benchrest shooters typically will
bring only primed brass (if that) to the range, and use hand
dies to carefully craft the ammo as it is shot.  In this manner,
it can be adjusted for the appropriate conditions (even down
to temperature compensation).   This takes great care in observing
effects of each shot and keeping careful records and having very
accurate reloading equipment.

Once new brass is fire-formed in this manner, it will only be
neck sized from then on, and only about .002" - .003" just so it will
put enough tension on the bullet to hold it in the neck.  
The resulting cases fit the chamber exactly and are inserted
(using the headstamp as a guide) in the same orientation as 
they were fire-formed in as they are subsequently realoaded and shot.
In normal benchrest shooting with properly fire-formed brass,
different rifles like the bullets different distances off the rifling,
but this will typically fall in the .005" to .015" range.  The
resulting cartridge lengths usually tend to exceed SAAMI specs
on this, but are perfectly acceptable if you are shooting
one shot at a time from a benchrest.

WARNING!!!!   Unless you know exactly what you are doing, do not
		follow any of the above information since without
		specific knowledge of why, it can be very
		dangerous.

--
-----
	Dion Hollenbeck             (619) 455-5590 x2814
	Megatek Corporation, 9645 Scranton Road, San Diego, CA  92121
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