Armament Research Services has released its latest Research Note, authored by ARES Technical Specialist Jonathan Ferguson. Research Note 7 gives an overview of the concept of cartridge headspace (CHS), and its importance in the correct functioning of a firearm. ARES Research Notes are short form publications, designed to provide brief, accurate information on a single topic or concept. Below are some excerpts from the full note, which can be read here.
‘Headspace’, sometimes termed ‘Cartridge Head Space’ or ‘Cartridge Headspace’ (CHS), is the distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats. This point usually corresponds to the rim or shoulder of the cartridge case, and varies depending upon the type of cartridge being chambered.
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Due to the high pressures involved, precise measurement and setting of CHS is crucial to the safe and reliable operation of a firearm. Insufficient headspace will hinder or prevent the chambering of a round. A certain amount of play or ‘head clearance’ is required to account for variance in ammunition production and fouling of the working parts during normal operation.
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Traditionally, headspace is measured using the rim of a cartridge, in which case the weapon’s headspace measurement consists of the rim itself, plus any excess gap between case head and bolt face. Head clearance in this instance therefore comprises the headspace measurement minus the thickness of the cartridge rim.
A good example of incorrect headspacing and its effects comes from ARES Director N.R. Jenzen-Jones’ Following the Headstamp Trail, published by the Small Arms Survey last year.
“…the use of the .308 Winchester cartridge in firearms chambered for 7.62×51 mm might have adverse effects on the firearms themselves, due to the slightly longer headspace present in weapons chambered for 7.62×51 mm. The cartridges may also suffer case failures when fired from such weapons, including rupturing of the case walls or separation of the case head.”
Read the full content of ARES Research Note 7: Cartridge Headspace. Other ARES publications can be found here.
Good little article. However there’s a few things that I feel I should add:
Headspace is checked with two gauges, typically a go, and a no-go. The no-go gauge is too long, and if the action closes on the “no go” gauge, there is too much headspace. The go gauge checks for inadequate headspace, in that it is the proper length and the gun must close on it to be within tolerance. Confusingly there is often a third gauge that is sometimes used with machineguns called a “field” gauge, this measures the maximum allowable length and is intended to work like a no-go gauge. Generally any gun that will close on a field gauge should be returned to arsenal for rebuild.
There’s another thing to be aware of when it comes to headspace, pistol cartridges that index off the case mouth can sometimes drop too far into the chamber because case mouth crimp is excessive, in certain cases these guns will still fire, however it can create dangerous overpressure as the bullet must squeeze out between the case mouth (which is now held at a smaller diameter by the barrel’s throat). I’ve seen this issue most consistently with 9mm, but also seen it in .25ACP, .32ACP and .40S&W. Usually it’s a sign of inferior workmanship as it pertains to ammunition manufacture.
Thanks, Drew. We touch on gauges briefly in the full (PDF) note, and hope this short PDF will be a useful reference guide for the lay person. ARES does have several qualified armourers on staff, some of whom have delved a little deeper into CHS issues for specific projects (including issues faced by Syrian rebels using various commercial .308 cartridges in their FAL rifles). We’ve updated the post above.
Ares,
Thanks! The PDF mentioned gauging, but was otherwise nonspecific, I thought I would throw in.
One problem that can be encountered with FAL rifles, is using commercial .308 SAAMI spec ammunition, which is loaded to a higher pressure (60,000 PSI) vs the more mild loading of 7.61x51mm M80 Ball which has a PMAX of 50,000PSI. This can greatly exacerbate headspace issues.
Another interesting defect I’ve seen is AK derivatives with soft trunnion/barrel rivets. Under firing, these pins can deform and the barrel will slide forward in the trunnion until headspace becomes excessive and start causing all kinds of havoc.
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