hes@ccvr1.cc.ncsu.edu (Henry E. Schaffer) (06/05/91)
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? ]
hollen@UCSD.EDU (Dion Hollenbeck) (06/06/91)
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
uunet!megatek!hollen or hollen@megatek.uucp