I have been looking at this wondering if it is worth the time and effort to respond.
Let me start off with a axiom: The cartridge case is a gas seal. It is not structurally strong enough to hold the combustion pressures and must be supported. Secondly, it is always operating above yield.
Reloaders and shooters seldom have to consider the engineering that it takes to support the case. Only when a firearm is improperly designed, and that is rare, do we find out what headspace means from a design perspective.
One of the first issues about supporting the case is how much of the case is allowed to stick out of the chamber. Unless you are going to knock the case out with a cleaning rod, there needs to be relief for the extractor, a bit of beveling to align the case, etc. So some of the case head is going to be sticking out of the chamber and it is my contention that action strength is best considered from a case support issue. Actions that support the case well are “stronger” than actions that leave a lot of case head unsupported.
You see, the case is the weakest link.*
The 40 S&W is a good example of something that went back due to poor case head support.
http://thefiringline.com/forums/showpost.php?p=4918338&postcount=23
As you can see in the pictures early Glock 40 S&W chambers left more case head unsupported than later chambers. Too much of the thin case sidewall was being left unsupported and given the high pressures and thin case sidewalls of the 40 S&W, shooters were blowing case heads. If you Google this you can find a lot of pictures of blown sidewalls and case heads.
So, in my opinion, fundamentally, “headspace” is a measure of how much of the case head is sticking out of the chamber.
But, from a manufacturing and a home gunsmith viewpoint, how do you measure that?
Chamber headspace gages are made under the assumption that the action was properly designed with a proper amount of case head protrusion. This is something that is not discussed much by reloaders or shooters because they don’t have to consider this issue., but it makes measuring chamber headspace with a metal gage easy.
For us, with bottlenecked cases, headspace is a dimension between shoulder and base of a case. This is important and something that needs to be controlled in reloading. Cases can only stretch so much, lets say 0.006”. If a case stretches more than 0.006” than you run the risk of a sidewall separation. Hopefully we all know that with a bottlenecked case the shoulder blows out first and sticks to the front of the chamber. Then as combustion pressures rise, the case sidewalls stretch to allow the base to meet the bolt face. If the sidewalls have to stretch too much, the case will rupture.
Straight walled cases don’t have a shoulder, and for the GAP, the case head is sticking out of the chamber the same amount as the 45 ACP (I am assuming this) and assuming the GAP case head is the same construction as the 45 ACP, the chamber support is about the same. As long as the extractor holds the case against the breech face the firing pin can hit the primer and the cartridge will ignite.
You can understand that push feed mechanisms actually need a shoulder or something to stop the case from going up the barrel and being too far from the firing pin, but claw extractors will hold the things on the breech face regardless of chamber headspace.
This can be bad. A shooting bud of mine fired a 270 Win cartridge in classic M70 in 300 Win Mag. He did not notice the 270 Win cartridge rolling around on the bench, the claw extractor held it against the bolt face, and the case head blew because a 270 Win is much thinner than a 300 Win Mag.
* There are people who believe that the case is a load carrying member. They did not come up with this idea on their own, this was something they were taught. It all came about from the early 1900’s problems with the single heat treat 03 Springfields and the Army coverup of these problems. The 03 Springfield was properly designed to carry the full thrust of the cartridge case, but the single heat treat Springfields were not properly built. The Army built over 1 million structurally deficient rifles, kept them in service, issued them, sold them to Civilians, and whenever rifles blew up, blamed the blowups on the shooters. To give logic to this cover up, the Army created this theory that the case carries load and removes load from the bolt face. There was actually a method to this as at the time shooters were greasing their bullets, because the bullets fouled something awful. Grease eliminated the fouling. Instead of acknowledging that their defective rifles were at fault, the Army told people that grease in the chamber removed the friction between case and chamber (which is true) and thus, dangerously, increased bolt thrust, which is the lie,... sort of. In a properly designed rifle firing ammunition within specs, there is no problem with greased or oiled cases, but in a defectively designed or defectively built rifle, any bolt thrust is a problem. The Army was now able to scapegoat shooters and misdirect the problem they created. The authority of the US Ordnance Department is so high that no one has ever questioned this, the idea that anyone would so stupidly design a rifle to break at something less than the full thrust of the cartridge. Instead, you have generations of people faithfully adhering to the idea that the case is supposed to carry load, and there will be generations more will believe this in the future, such is their faith in the infallibility and inerrancy of the US Army Ordnance Department. In fact, even today, whenever any problem happens with an Army weapon, the first thing the Army looks for is an oil can to blame.
