Headspace (firearms)
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In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct positioning. Different cartridges have their datum lines in different positions in relation to the cartridge.[1] For example, 5.56 NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the rim of the cartridge.
If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the cartridge case may rupture, possibly damaging the firearm and possibly injuring the shooter.[1]
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[edit] Cartridge types
A range of small arms cartridges have a flange at the base of the case called a rim. A part of the firearm called an extractor hooks onto the rim to pull the spent case out of the chamber after it has been fired. The cartridge is said to be rimmed when the rim is of larger diameter than the remainder of the case. Most early cartridges were rimmed, and the rim-stop ledge at the rear of the chamber prevents the cartridge from moving forward. Headspace for rimmed cartridges is the thickness of that rim.[1].
The cartridge is said to be rimless if the extractor groove is machined into the body of the case so the rim is the same diameter as the adjacent part of the case. Most modern automatic weapons use rimless cartridges. Straight rimless cartridges may headspace on the forward edge of the cartridge as shown in the diagram at the top of this article. Bottle-necked rimless cartridges may headspace on the conical shoulder between the narrow neck and the larger diameter portion of the case.[1]
Some large rimless magnum or military cartridges have a belt machined above the extractor groove. This belt is of slightly larger diameter than the adjacent case, so the cartridge may headspace on the edge of the belt closest to the bullet.[1]
[edit] Theory
A firearm cartridge cases serve two functions. 1. Provides a container for the powder, bullet, and primer, and 2. On firing the expanding case seals the breech of the firearm to prevent burning powder gas from escaping rearward. Most cartridge cases are made of brass and are designed to expand under pressure to form a close fit with the sides of the chamber and then contract so that the case can be removed. Centerfire cartridge cases are thicker near the base to prevent expansion into openings between the rear end of the barrel and the action where the cartridge case is not fully enclosed or supported by the chamber.[2]
When a firearm has more headspace than the cartridge design anticipated, the action or firing pin may move the cartridge forward to leave space between the chamber face of the action and the base of the cartridge. Pressure of burning powder gasses expands the thinner forward walls of the cartridge case to firmly grip against the sides of the chamber preventing rearward motion. The thicker base of the cartridge case (or sometimes the primer) may then be forced back into available space at the rear of the chamber. Where rearward movement exceeds cartridge design, the primer may rupture or the base of the cartridge case may begin to separate from the forward walls. Either type of failure may release high pressure gas and fragments being expelled from the action of the firearm. These may cause burns, cuts, or eye injury to the shooter or persons standing nearby.[1]
Headspace is a critical consideration in any firearm. Have it checked by a competent person using the appropriate headspace gauges prior to using the firearm.
[edit] Headspace gauges
Headspace is measured with a set of two headspace gauges; a "Go" gauge, and a "No-Go" gauge. Modern headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a "Go" and a "No-Go" gauge are required to headspace a firearm properly.[1]
Headspace gauges are typically used by inserting the gauge into the firearm chamber. The bolt should close and lock on a "Go" gauge, and not close on a "No-Go" gauge, indicating that the chamber headspace of a firearm is within safe minimum and maximum dimensions, respectively. The force that is applied to the bolt on a bolt-action firearm when making these assessments should only be at normal levels of force; otherwise, an incorrect assessment of headspace may result if the bolt is forced into a position with excessive pressure.[1]
[edit] Other cartridge dimensions
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Headspace is not the only important cartridge dimension. Other dimensions like overall cartridge length and base diameter are also significant for proper fit of a cartridge into a firearm, and may be included as part of a headspace gauge. Cartridges very close to chamber dimensions generally offer superior accuracy, but slightly undersized cartridges load more smoothly and reliably under variable temperature conditions or when fouling accumulates in the chamber or cartridges become corroded.
A firearm chamber for rimmed ammunition will often have two small ledges or steps. One at the rear engages with the forward face of the rim, as the rim-stop. The other step in the front of the chamber accommodates the thickness of the case wall at the forward edge of the case neck where the projectile emerges as a close fit to the beginning of the bore, the throat or leade. If the case length is too long, then the bolt on closing pushes the case over the forward step of the chamber at the throat / leade and can crush or crimp the case around the projectile at the neck of the case leading to all sorts of performance, pressure and safety problems.
Many target shooters who use rimmed ammunition, such as .303 British, will carefully sort the cases by headstamps into groups of the same make or batch, then gauge the thickness of the case rim into lots of the same, or very close, rim thickness. They then keep the fire-formed cases which are now optimised in shape to that firearm chamber, thereby increasing the accuracy potential by only having to resize the neck of the case and to trim-to-length so that the batch performance of the reloaded ammunition is as consistent as is possible from shot to shot. Some competition reloaders leave the primer slightly proud of the case by a known amount that is taken up when the bolt is locked home. This mechanical action on the center of the case base via the primer, as it is pushed a very small amount further into its cup, translates into an even distribution of pressure through the rim to the rim-stop. This also gives the advantage of the firing pin indenting the primer in a similar fashion from shot to shot as the primer cup is in direct contact with the bolt face. As with any reloading all care must be taken and manufacturer's recommendations followed.
[edit] External Articles
Cruffler.com article from October 1999
Forster Products article
How to measure headspace PDF paper of Armalite.com
