# headspace



## Militant_Tiger (Feb 23, 2004)

what exactly is it?


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## Bobm (Aug 26, 2003)

The closet in Clintons' office


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## sdeprie (May 1, 2004)

C'mon, bobm. You know he didn't use the closet. 
Headspace is the space between the bolt and the front load bearing part of a cartridge holding it in exactly the right place when being fired. Rimless and beltless cartridges headspace on the shoulder. It is far enough forward to press against the shoulder in the chamber to hold the cartridge against the bolt. Belted cartridges use the belt to do the same thing. If the shoulder is set too far back on them the cartridge will still be held against the bolt. When fired the cartridge will stretch to fill the space, but will not be as much of a danger of head separation. Most rimmed cartridges do the same thing with the rim, with or without a shoulder. If your headspace is too deep in a rifle, the cartridge may be allowed to slide too far forward, making a light primer strick which may cause a misfire, or it may fire and suddenly stretch the whole cartridge and may cause dangerous case separation with escaping hot gasses, hot metal fragments, etc escaping right into someone's surprised face, uncomfortable and dangerous. I may not have stated it perfectly, but this is the basic concept of headspace. Does that help, or were you asking something different. If you were asking something different, never mind.


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## OneShotOneKill (Feb 13, 2004)

There are two ways to look at headspace. First, let's consider the practical definition, and follow up with the technical one.

In practical terms, headspace is the clearance allowed between the base of the cartridge case and the face of the bolt. The position of the cartridge in the chamber is controlled in a variety of ways depending on the type of cartridge case involved. Most rimless automatic pistol cartridges (e.g., 9x19mm Parabellum, .45 ACP) are positioned by the case mouth resting on the front edge of the chamber. Rimmed cartridges (e.g., .45 Colt, .30/30 Winchester) are positioned in the chamber by the face of the rim resting on the rim recess at the back of the chamber. Belted magnum cases (e.g., .300 Win Mag., .458 Win Mag.) are positioned by the belt resting on the recess provided for it at the rear of the chamber, much as rimmed cartridges are. Finally, bottle necked rimless cartridges are positioned by the shoulder of the case resting against the shoulder of the chamber.

So when a cartridge is chambered, it is positioned in one of the above ways. Once positioned, there must be a little free space between the cartridge case base and the face of the bolt, to allow for dimensional tolerances in cases. When the cartridge is fired, the case expands in all directions, including toward the bolt face. If there is the intended clearance space, the case head is not stretched excessively during this expansion. If the space is too much, as the case head is pushed backwards towards the bolt face it may stretch enough for it to significantly weaken the case in the area just in front of the thick portion of the case head, called the web, where the thinner walls of the powder containing part of the case begin. If the case head does not separate on the first firing, the weakened brass may do so on subsequent firing. This is a very bad thing, as hot gas at 50,000 psi will damage at least the stock and magazine, if not the firer's hands or face. Little drops of molten brass and brass shards are carried by the hot gas at near-supersonic speeds. If all the firer gets is a Chicken Pox-like tattoo, he or she is fortunate.

Conversely, too little clearance is a bad thing, too. That is, if there is "negative" clearance and the case has to be forced into the chamber by the bolt, it can wedge the case neck tightly around the bullet, raising pressures by thus delaying bullet release. Then the hot gas will come out the primer pocket...

The technical definition of "headspace" for bottle-necked rimless cases is the dimension between the bolt face and the datum line on the chamber or cartridge shoulder, whichever is being referenced.


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## sdeprie (May 1, 2004)

Isn't that, more-or-less, what I said? Whatever. Tiger, is that clear?


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## OneShotOneKill (Feb 13, 2004)

Always keep an eye on your head spacing in all your firearms! Check your case trim length every time! The 357 Sig has a shoulder but head spaces off the case mouth and uses .355 dia bullets.


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