My question is, what do you do if your cheap surplus rifle demonstrates moderately excess headspace? Enough to show on a gauge but not enough to give casehead separation on cheap surplus ammunition.
Assess the shooting risks and decide whether to shoot the thing.
I have been studying this for some time and headspace is not an easy topic to discuss. People have fallacious theories on the cartridge case, many believe that it does not, can not, deform. Their expectations are that it is a strong pressure vessel and how much hangs out of the chamber does not matter. However, these same people have contradictory beliefs when it comes to setting the shoulder back on bottlenecked cases. They do believe that pushing the case shoulder too far back will result in case head separations, and therefore you would think they understand the case is weak, but the contradiction is they don’t believe the case head will burst with excessive case head protrusion.
I have searched on the web for numbers for case head protrusion, but the numbers are not out there. This is the most safety critical issue for headspace, but tables of acceptable case head protrusion are not to be found anywhere. Historically, gunsmiths determined acceptable case head protrusion by taking measurements from factory guns. They took measurements of the barrel shank, the receiver ring, subtracted distances, and then after sticking a chamber headspace gage into the chamber, measuring to the bolt face.
I believe the ignorance of this issue is because this is something we shooters do not have to worry about this. The system was designed so that trained monkeys could drop a metal gage into a chamber, and if the gage was not between Go or NO Go, the monkey was trained to take the firearm to a higher authority for assessment. Basically, if the monkeys use the provided gages, use the parts given them, and follow procedures, the firearm will assembly correctly. Notice that this assumes the parts are properly dimensioned. If the monkey’s start removing material off the back of bolt lugs, remove material from receiver seats, cut the chamber cone wrong, or port that 45 ACP barrel too much, then the parts are no longer within factory tolerances and case heads are sticking out more than they should.
It is rare that a factory messes up head protrusion, but it happens. Back in the mid nineties 40 Caliber Glocks were blowing case heads and you can find threads on this. There was a lot of denial about this too, again from people who believe that cases are strong, so strong that they don't need to be supported by the chamber.
There is a wonderful discussion of the importance of case head protrusion in Vol IV of the Machine Gun by Chinn. These pictures come from the section on blowback. Everyone should read this section, because Chinn does such a good job explaining that controlling case head protrusion is absolutely critical in firearm design. And how it effects the design approach.
Notice that case head design limits the amount of allowable movement of the cartridge case.
For this bottlenecked system to work, the cartridge had to be heavily greased. The British Polsten automatic cannon used greased rounds, operated on advanced primer ignition, and used this basic concept.
Timing and just at what pressures are at unlock are critical to gas mechanisms:
Belted magnums are the most confusing system out there and everyone will have in time, case head separations with these things. While all factory rifles have correct cartridge head protrusion, but between different manufacturers, the shoulder to base distance all vary. The shoulder to base distance is not controlled as these things headspace off the rim. This results in very short case life, the most common problem is case head separations, especially after the first firing as the factory case shoulder is usually very far from the chamber shoulder of the firearm. For belted magnums I lubricate the heck out of the case. This ensures that the case slides to the bolt face without any sidewall stretch. The case shoulder simply folds out to the chamber shoulder establishing proper base to shoulder headspace without creating necking in the case sidewalls, which would have happened if the case and chamber were dry. After first firing I bump the shoulder back 0.003” and use those special Sinclair cartridge headspace gages. There is an excellent discussion of this at
http://www.realguns.com/Commentary/comar46.htm “Extending Cartridge Case Life” . The damnable thing is because base to shoulder is not controlled, the die has to be set up for one particular rifle: two rifles in that cartridge would need two unique sizing dies, or you would have to spend a huge amount of set up time sizing ammo from the other rifle.
The US Army, (Air Corp and land units) and the US Navy used the 20mm Hispano Suiza from WW2 through Vietnam. After WW2 automatic oilers were installed, but during WW2 the automatic cannon used pre-greased cartridges and because of the grease, they got lucky. As Chinn discusses here, the grease saved them!
The Machine Gun, Vol 1 LTC Chinn, 20mm Hispano-Suiza page 589
http://www.milsurps.com/content.php?r=347-The-Machine-Gun-(by-George-M.-Chinn)
Thus the most vital measurement (headspace) in any automatic weapon was governed by chance in this instance.
An unfortunate discovery was that chamber errors in the gun could be corrected for the moment covering the ammunition case with a heavy lubricant. If the chamber was oversize, it served as a fluid fit to make up the deficiency and, if unsafe headspace existed that would result in case rupture if ammunition was fired dry, then the lubricant allowed the cartridge case to slip back at the start of pressure build up, to take up the slack between the breech lock and the breech lock key. Had this method of “quick fix” not been possible, the Navy would have long ago recognized the seriousness of the situation. In fact, this inexcusable method of correction was in use so long that it was becoming accepted as a satisfactory solution of a necessary nuisance. 
[/IMG]
