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From: hes@ccvr1.cc.ncsu.edu (Henry E. Schaffer)
Newsgroups: rec.guns
Subject: Rifle Chamber Measurements (Part 1)
Message-ID: <35247@mimsy.umd.edu>
Date: 5 Jun 91 14:27:48 GMT
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Organization: NCSU Computing Center
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  I decided to make some measurements on my .308 Win. rifle chamber
to better understand safety and accuracy, particularly regarding 
reloading.  However I didn't want to take the rifle apart, or even 
to make a chamber cast, so I bought a couple of gadgets to help me.

  The first gadget is the small Sinclair Chamber Length Gage Insert.
This is a small flat-topped mushroom shaped piece of leaded steel.
The top is just the diameter of the cartridge case neck, and the
stem is just barely big enough to fit inside the cartridge case neck
and be held firmly.  (Sinclair says that the leaded steel is to 
make sure that the insert can't damage the chamber - but my friend
in the materials department says it might rather be to make the
steel more easily machinable - or perhaps both.)

  You take a cartridge case, full length resize it, drill out the
primer hole (I'll tell you why later) and trim back the neck .1".
Then put the gage insert into the neck so that it is longer than
a normal cartridge case.  This is the working gadget.  It is then
put into the rifle chamber and the bolt is closed, and then opened
slowly and carefully.  The head of the insert will have contacted 
the chamber end, forcing the insert back into the case.  Then measure
the length from the base of this gadget to the top, and you'll have
the chamber length.  SAAMI specs say that the case trim length must 
be at least .024" less than the chamber length.  (Then to reset the
insert it is necessary to *push* it back out of the case neck so
as to be longer than the chamber length.  Now you see why the primer
pocket was drilled out!  A pin punch can be inserted, pressed
against the bottom of the insert and tapped with a mallet.  The
instructions say to "push the case insert", but actually I had to
hit the pin punch kind of hard with a light hammer!)

  In my Remington 700, the chamber measured 2.0465", and so this
means that my cases must always be 2.0225" or less.  The SAAMI
spec for case length of the .308 Win is 2.015" (I assume that this
is the maximum length), and my Lee trimmer trims back to 2.010".
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!)  New
cases are often in the 2.002" - 2.012" length, and gain perhaps
.005" after firing.  The necks should elongate after reloading
and firing again, with supposedly less elongation when using
neck sizing then when full length resizing.  The amount of
elongation should be less for milder loads, and greater for hotter 
loads.  (I think that the amount of neck elongation due to full 
length resizing is also increased if there is greater headspace.)  
Sinclair does not recommend going beyond the SAAMI case length spec, 
regardless of the chamber measurement.  They say "Excessive case 
length will cause DANGEROUS chamber pressures resulting in personal 
injury.  We do not recommend increasing case length beyond 
published SAAMI specifications.  DO SO AT YOUR OWN RISK!"

  Since a new case which is at the SAAMI maximum spec (2.015) will
get uncomfortably close to the maximum allowable length after only
one firing - this indicates to me that measuring case length is a
very important safety step.  I generally measure case lengths by
putting them in my trimmer and seeing if it takes off any brass.  If
it does, then I gently chamfer both the inside and outside edges of
the case neck.  (After trimming, I don't bother to measure again 
every reloading, but do so sometimes.)

  I've heard that rifle chambers are generally made to be fairly
generous in length, it could be that this will not often be a
problem.  (I have reason to believe that my rifle's chamber is
close to minimum specs in size - I'll discuss this further below.)
However, this is something hard to tell without actually measuring
the chamber.  So now I have much better knowledge of my chamber
length.  This measurement tells me that I don't have very much
leeway in case length, and that I need to take case trimming
quite seriously.

  Sinclair makes this gage insert in several calibers - note that
only the diameter is important, so the .308 insert can also be
used in the .30-06, .300 Win, .300 Wby, .308 Norma, etc.  When
you buy this, you get this little gage insert, and a small page
of instructions - but that was all it took.

  After this, I got to work with my new RCBS Precision Mic set,
which is a very neat gadget (really a small gadget set) - I'll
cover that adventure in the next installment.

--henry schaffer  n c state univ

[MODERATOR:  A nice summary from Henry, as always, thanks.  Could
you pass along where you got the insert and how much it, err, set
you back? ]