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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. 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.
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.
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.
Some large rimless magnum or military cartridges have a belt formed 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. The original purpose of the belt was to give accurate headspacing for cartridges with shallow shoulder angles, where longitudinal precision of seating using such a shoulder presents difficulties. In effect this was similar to the headspacing function of a rim, but gave a long enough surface for cartridges to lie side-by-side in a magazine without risk of interference during the feed-stroke. The adoption of the belt as headspacing feature on rounds such as the .300 Holland & Holland Magnum and its derivatives ultimately started a fashion that resulted in most later Magnum rifle cartridges featuring belts.
Some more or less straight cartridges have no bottleneck to headspace on. Such cylindrical shaped cartridge cases use the case mouth (the forward end of a cartridge case) as a forward positioned flange used for headspacing.
C.I.P. ammunition approval procedure and headspace
The standards organization Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (Permanent International Commission for Firearms Testing - commonly abbreviated as C.I.P. or CIP) uses a rimless, rimmed, magnum, pistol and revolver and rimfire cartridges classification to differentiate between hundreds of different small arms chamberings. The dimensional checking of the ammunition, as described in the C.I.P. Decisions, Texts and Tables, is left to the manufacturers themselves. During C.I.P. ammunition approval headspace is not checked, even on rimless cartridges with taper. The view is that in the very unlikely case (according to the current quality standards) the cartridge is too long, once pressed by the bolt, the pressure will rise too high causing rejection. If it is too short, firing will fail also causing rejection.
A firearm cartridge case serves two functions. The most obvious role is to provide a container for the powder, bullet, and primer. Additionally though, upon 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.
When a firearm has more headspace than the cartridge design anticipated, the closing of the bolt or impact of the 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, causing the case to stretch. 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 case fragments from the action of the firearm. These may cause burns, cuts, or eye injury to the shooter or persons standing nearby.
Headspace is a critical consideration in any firearm. It should be checked by a competent person using the appropriate headspace gauges prior to using the firearm.
To gauge 'headspace' its meaning must first be understood. It is the space remaining between the case head and the bolt face when the bolt is closed thereby locking the breech. The cartridge case must find a surface to push up to but not such so as to be firmly or tightly bound so that expansion of the cartridge case cannot occur. It is the expansion and contraction characteristic of brass that makes it an ideal metal for use in firearm ammunition.
There are four 'types' of headspace control to consider based on the case typ used;
1. For a rimmed case where the rim extends beyond the diameter of the case such as with .22LR, 30-30 and .303 Brit as examples, it is the forward or front face of the rim that is stopped by a ledge at the very rear of the chamber and it is from this datum to the face of the bolt that headspace is measured for rimmed ammunition. If the headspace equals the rim thickness then no space is available for the head of the case to move into as the case body or shank expands and contracts thereby making it difficult to extract the case after discharge. Competition target shooters will often carefully sort their empty cases by rim thickness into like lots as an aid to more accurate ammunition bought about by consistent reloading making each round as similar as is possible to the other.
2. For a straight walled case (which is ammunition without a shoulder and elbow tapering down to form the neck), such as with pistol (9mm or .380ACP) or some rifle ammunition (M1 30cal carbine), it is at the case mouth that a ledge is encountered in the front part of the chamber stopping forward movement of the loaded round. The projectile fits tight within the case mouth and it is the thickness of the case wall that allows for this type of headspacing. This ammunition type must not be roll crimped as the rounded edge of the case mouth will not engage safely with the stop ledge, this is why a light taper crimp is often used if considered necessary.
3. Rimless cases actually have a rim but it is usually constrained at the same diameter as the widest part of the cartridge case (.308 Win, .223 Rem). This style of case relies on the sloping face of the case between the shoulder and elbow to stop forward movement of the round within the chamber. It is from this face, as a datum, that the distance to the bolt face is considered in measuring headspace.
4. A belted case has a raised band just in front of the rim and extractor groove. It is the front face of this belt that is used to stop forward movement of the loaded cartridge against a ledge within the rear of the chamber working in a similar way to that of headspace datum for a rimmed case. This type was developed for large powerful cartridges used for the most dangerous game as this type has proven most reliable.
Headspace is measured with a set of two headspace gauges; a "Go" gauge, and a "No-Go" gauge. 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 for a gunsmith to headspace a firearm properly.
A third gauge, the "Field" gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the "factory specs" measured by the "go" and "no-go" gauges. A bolt that closes on "no-go" but not on "field" is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec.
Headspace gauges are typically used by inserting the gauge into the bolt face engaging the extractor claw and then into the firearm chamber, and noting which gauge the bolt fully closes upon, and which it does not. The closing force applied to the bolt on a bolt-action firearm when making these assessments should be consistently and lightly applied; otherwise, an incorrect assessment of headspace will result if the bolt is forced close with excessive pressure.
The below table illustrates the function of each gauge:
|Gauge||Measurement||If Bolt Closes on Gauge||If Bolt Does Not Close on Gauge|
|Go||Minimum factory-spec spacing||"Go". Headspace is greater than minimum spec.||"No-Go." Headspace is too short. Bolt will not close on factory-spec cartridges, resulting in jammed brass, and firing out-of-battery, both dangerous conditions.|
|No-Go||Maximum factory spec spacing||"No-Go." Headspace is greater than the maximum factory spec. Risk of case ruptures if used.||"Go". Headspace is below maximum factory spec.|
|Field||Absolute maximum safe spacing.||Rifle is unsafe to fire. Headspace is greater than what is considered safe to fire. High risk of case rupture if fired.||Generally "go". Headspace is below the maximum for what is considered safe to fire when using factory spec ammunition. Rifle should be evaluated by a gunsmith for possible re-headspacing.|
Other cartridge dimensions
|This section does not cite any references or sources. (May 2011)|
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.
- Davis, William C., Jr. Handloading (1981) National Rifle Association pp.67-69 ISBN=0-935998-34-9
- C.I.P. HOMOLOGATION List (decisions, texts and tables) of TDCC
- Davis, William C., Jr. Handloading (1981) National Rifle Association pp.8-11 ISBN=0-935998-34-9
- Cruffler.com article from October 1999
- Forster Products article
- How to measure headspace PDF paper of Armalite.com
- C.I.P. HOMOLOGATION List of TDCC - Tab I - Rimless cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab II - Rimmed cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab III - Magnum cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab IV - Pistol and revolver cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab V - Rimfire cartridges - Crusher
- C.I.P. HOMOLOGATION List of TDCC - Tab V - Rimfire cartridges - Transducer
- C.I.P. HOMOLOGATION List of TDCC - Tab VI - Cartridges for industrial use
- C.I.P. HOMOLOGATION List of TDCC - Tab VII - Shot cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab VIII - Cartridges for alarm weapons
- C.I.P. HOMOLOGATION List of TDCC - Tab IX - Dust shot cartridges
- C.I.P. HOMOLOGATION List of TDCC - Tab X - Cartridges for other weapons