From: njohnson@marlin.nosc.mil (Norman F. Johnson)
Newsgroups: rec.guns
Subject: Re: Headspace
Date: 8 Aug 1996 10:13:02 -0400

# How exactly is "headspace" defined. I keep hearing this term. I 
# hope it is not a religion. Also, "headspace on the mouth", 
# "headspace on the shoulder", etc?

Headspace is a distance from some datum to the face of the closed 
bolt.  

On a rimmed cartridge it is from the face of the cutout in the 
chamber (that which surrounds and encloses the case rim) to the 
closed bolt face.  For instance, a hypothetical rimmed cartridge 
with a .050" thick rim will have zero headspace CLEARANCE if bolt 
to chamber rim face (headspace) is .050".  A degree of "slop" is 
allowed to accommodate manufacturing tolerances of cases and 
chambers.  Specs might call for a headspace of .053" -.004", 
+.004".  These are made up numbers as I do not have my set of 
SAMMI specs here now but they are not too far off.  

On a rimless BOTTLENECK cartridge, headspace is a linear measure-
ment from the closed bolt to a datum line that is located some-
where on the shoulder-to-neck slope of the cartridge.  

On a rimless STRAIGHT case it is measured from the bolt face to 
end of the straight portion of the chamber where the case mouth 
is located. 

One can see that headspace is measured in THOUSANDTHS OF AN INCH 
on rimmed cases and in INCHES on rimless cases.  Allowance for 
clearances, however, as discussed above, is about the same for 
either, a few thousandths of an inch.  

To check the headspace clearance on a bolt action rifle, I use 
this method: 

1)  use a new case or completely pull down a factory loaded round 
    so that the case may be used. 

2)  start new primer into case about 1/2 way 

3)  chamber the case by hand 

4) close the bolt gently (this will not detonate the primer) 

5)  extract the case carefully 

6)  with a depth gauge measure the amount of primer protrusion 

I have used this method for years with satisfaction.  

This method of headspace clearance measurement is not as good as 
using a headspace gage because of manufacturing tolerances of the 
brass, but it will show you any gross dimensional problems and is 
certainly close enough to demonstrate the benefits of fire formed 
cases.  

A special note:

Theoretically the .45 auto headspaces on the case mouth but any 
one who has ever measured the length of .45 brass and/or the 
length of .45 auto chambers knows that this is not the way it 
works in the real world.  The brass is typically short and the 
chambers are typically long.  

The Browning design is marvelous in that it allows for manufac-
turing sloppiness and still comes out firing.  

In this case it is because the genius Browning came up with an 
extractor that holds the case close enough to the slide face that 
the firing pin will hit the primer hard enough to fire every 
time, a very necessary quality for a close range military weapon.  
This is true of even VERY short cases.  
 
However, even tho it will fire dependably, the accuracy is not 
likely to be as good as it would if the primer were held in the 
same position relative to the firing pin each time.  Weak 
(varying) firing pin blows can cause inconsistent primer 
performance and subsequent erratic ignition and accuracy.

For the 1911 and its clones, an old trick is to seat the bullet 
so that headspacing is accomplished by cartridge OAL.  The bullet 
is seated so that it just touches the beginning of the rifling 
when chambered and thereby establishes a virtual zero headspace.  

To do this for the 1911:

1. Remove the barrel from the gun and make sure the barrel/cham-
ber are squeaky clean.

2. Load a dummy round (using a bullet from the batch that you 
intend to load) so that the cartridge OAL is quite long and drop 
it into the chamber.  Note the approximate length that it stands 
above the lip that projects beyond the rest of the barrel.  

3. Gradually adjust your seating die so that the cartridge, with 
gentle pressure from your finger, is exactly flush with the end 
of the lip.

That's it!

You may find, depending upon your particular gun that accuracy is 
appreciably improved.  

I have used this technique with just about every bullet shape in 
existence, including flat wadcutters which my old rattle trap 
1911 house gun will feed reliably.  In my .45's at least, the OAL 
is not too long to fit the magazine or to feed reliably.  If you 
get these cartridges too long, the action will not lock up and 
the gun will not fire.

God Bless!

Norm