Workbook

You don't want to buy a gauge? You really should. Get a factory loaded Federal 168g or 173g

National Match .308 cartridge or unfired brass. This is your GO gauge. Cut a disk of masking tape

or a feeler gauge that fits over the base of the cartridge. The case plus tape is your NOGO gauge

(that's right -- the thickness of a piece of tape is the only variance allowed). You must not use

anything but National Match ammunition. Typical military 7.62x51mm is slightly undersize for

reliable feeding in belt-fed machine-guns that can handle the variances.

WHAT IS SAFE FOR MY FAL?

A .308 headspace dimensions are 1.630" minimum (GO) and 1.634" maximum (NO GO). The

difference is .004" which creates a problem. The FAL must have interchangeable parts. The weapon

needs to be factory assembled without hand-fitting parts. The locking shoulder moves slightly

("setback") during initial firing. These variations are cumulative. The amount of tolerance needed

follows.

Interchangeability of parts 0.0045"

Headspacing 0.0010"

Locking shoulder set back (.0007 to .0022) 0.0015"

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Factory tolerance 0.0070"

Field wear tolerance 0.0015"

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Total headspace 0.0085"

Tolerances are the sum total of the maximum error in each of seven categories. I doubt any weapon

would display maximum variance in each category, but it is theoretically possible. Our acceptable

variance is now 0.0085." This is far greater than the .0040" variances of our gauge. Because of this,

the Rifle Steering Committee ordered extensive evaluation of tolerance variations and safe operating

dimensions and came to the following conclusions.

"A" dimension is from the .400 point on the cartridge shoulder to the bolt face (headspace). "B" is

from the .400 datum to the locking shoulder face.

U. S. GO and NOGO gauge A 1.6300" to 1.6340" (.0040")

Factory new rifle, test fired A 1.6315" to 1.6385" (.0070")

B 5.4475" to 5.4500"

Rifle in Service A 1.6315" to 1.6400" (.0085")

B 5.4475" to 5.4515"

Given the predictable wear of parts and the locking shoulder set back, I like to headspace all barrels

to .001" under where I want to end up, which is .002" over minimum, or half way between a GO and

a NO GO. I do not recommend setting for minimum headspace. I find the popular press extolled

virtues of a "minimum" headspace to be highly overrated, and the increased reliability of slightly

looser headspace to be valuable, considering the wide range of quality in the available surplus

ammunition market. Consider this. The Gunwriter-Whores* say selecting a service rifle with

minimum headspace is a good thing. But what chamber reamer cuts the smallest chamber? That's

right – the oldest, most worn one. So a new razor sharp reamer will cut a perfect chamber, and a

dull, worn-out reamer will cut a sloppy chamber, perhaps with chatter marks and scoring, but it will

be "tighter." I'm not impressed. By the way – my "loose chamber" philosophy is for service grade

autoloaders, not precision bolt guns or rifles restricted to special ammunition.