From: cjp@megatek.com (Chris "Systems Stud" Pikus)
Newsgroups: rec.guns
Subject: Re: AR15 Assembly / NESARD Kits
Date: 19 Nov 92 15:09:03 GMT
Organization: Megatek Corporation, San Diego, California


	[Perhaps this (or something like it) should be an FAQ]

CLF@ECLX.PSU.EDU (Christopher L Frame) writes:
# 
# 	I was reading my copy of Duncan Long's "The AR15/M16 A Practical Guide"
# and came across a confusing series of statements about headspace gauges and
# their proper use.  I quote from pg 80:
# 
# 	"If the chamber allowed the bolt to close on the no-go gauge but not
# the field gauge, it would be safe to fire the rifle with regular loads."
# 
# 	and one sentence down:
# 
# 	"Whenever possible, buy only a rifle or barrel that accepts the go and
# field gauge but not the no-go gauge."
# 
# Aren't these statements contradictory with respect to the FIELD gauge?
# 
# Christopher Frame

	It seems to me that the above contradiction is mere something that
slipped past the editor. Instead of just giving a simple answer, permit
me to explain.


	Headspace guages are used to measure the size (acutally length)
of the chamber. They work in a binary fasion: either the bolt can close
on it or it cannot. There is no need to worry about measurement error (as-
suming that you have a good set of guages.)


	The "GO" guage measures whether the chamber is too small. It is
sized to at the small end of the factory tolerance range. The bolt MUST
close on it. If it does not, then the chamber is too small.

	The "NO-GO" guage measures whether the chamber is too large. It
is set to the large end of the factory tolerence range. The bolt should
_NOT_ close on this guage. If it does, that means that the chamber is
outside of factory tolerance.

	The "FIELD" guage is even larger still. The bolt should _NEVER_
be able to close on it. If so that means that the gun is unsafe.

	So what's the difference between NO-GO and FIELD? It's the
difference between factory and end-of-life tolerances in machined
parts: there are tolerances for new/rebuilt, and wider tolerances to
tell you when to throw something away. Since the wear mode on rifle
chambers is to stretch, then we only have to measure for too large.

	Thus, gunsmiths who work on your barrel/chamber will use the
GO/NO-GO to determine if it is within factory new/rebuilt tolerances.
You, out in the field _could_ just use the FIELD guage to determine
when to throw out the barrel, but I would reccommend against it. Use
the NO-GO instead.

	Personally, I do not want to fiddle with the FIELD guage, I
use the NO-GO guage to determine end of life. It's a bit more con-
servative, but it gives me a bit more safety margin.

	OK, so maybe I'm a wimp. But, unless you shoot incredible quan-
tities of ammo (>5,000 per year), you probably wouldn't miss much barrel
life anyway.

	Based upon the above, you can probably infer which statement
in the book is correct.

	P.S. There are rumors that certain "aggressively priced" re-
importers of US surplus rifles use the FIELD guage instead of the NO-GO
guage so that they can get higher yields of "shootable" firearms.






-- 
Christopher J. Pikus,        | "A lot of times men do things they don't want to 
Megatek Corp., San Diego, CA | do so the women they are going out with will do
INTERNET: cjp@megatek.com    | things _they_ don't want do do."   -- Tim Allen