Few tolerances found on a firearm are of more importance than headspace. Not only is headspace essential to rifle accuracy, but it is a major safety factor to consider when assembling a firearm. Improper headspace can cause case head separation, unsafe chamber pressure levels, or failure to function.
What is headspace?
Headspace is the distance measured from the bolt face to the portion of the chamber restricting the cartridge from further forward movement. For most modern rimless centerfire rifle cartridges this datum point is found on the shoulder of the chamber. For most rimmed rifle cartridges headspace is set off of the forward portion of the case rim itself. Belted magnums typically headspace off the front most portion of the belt, while non-rimmed straight walled pistol cartridges will headspace on the case mouth.
What happens if headspace is too short?
If headspace is left too short it is most likely that a cartridge will fail to feed into battery properly. With most modern rifle designs this will render the firearm inoperable (not all designs require a firearm to go all the way into battery before allowing the firing pin to release and strike the primer). If enough force is exerted while forcing a round into a short chamber the case neck will be jammed forward into the throat. This deformation of the cartridge could result in excessive chamber pressures during the discharge of the round.
What happens if headspace is too long?
A number of undesirable consequences can occur if headspace is set too long. Aside from increased variation in feed angle and bullet alignment (and a subsequent decrease in accuracy potential), unwanted deformation will occur to the cartridge case and primer.
One good indicator that a chamber/bolt combination is experiencing excessive headspace is a protruding primer after discharge (with proper headspace the primer should remain flush with the base of the case). Excessive headspace may cause bulged cases, cracks in the case body, or splits in the case neck.
In extreme instances of excessive headspace complete case head separation can occur. This may result in damage to the firearm as well as possible injury to the shooter. While injury to the shooter is unlikely with many modern platforms such as direct impingment AR-15’s, there are a number of other rifle platforms where these additional safety features are not inherent in the design of the firearm. It is for this reason that we recommend shooters check their headspace on all rifle builds when purchasing their bolt or barrel from different suppliers. While Criterion may have control over the headspace tolerances of our barrels, we cannot guarantee that bolt manufacturers will hold their products to the exact same specifications. Standards such as those laid out by SAAMI and corresponding military specifications may help with some rifle patterns, but it is still highly recommended that headspace is checked with each and every new bolt/barrel combination used.
What factors can lead to improper headspace?
A number of factors may be responsible for improper headspace. Some of these include the following:
Some of these discrepancies may simply be caused by normal wear and tear of different components. Well-worn bolt lugs will likely feature a different headspace specification when compared to a new-in-box bolt. Some bolts may even be sold slightly oversized with the intent to have the installing armorer lap the bolt lugs to the receiver for proper fitment (as is often the case with M14 bolts).
Unique Manufacturer Tolerances
While some platforms like the AR-15 are held to a certain “mil-spec” tolerance. Other rifle platforms (including most .308 AR’s) do not share a uniform tolerance across the firearm industry. While Criterion has sought to set our tolerance specification to be cross compatible with most major manufacturers of DPMS pattern .308 AR’s (DPMS, Fulton Armory, JP Enterprises, Midwest Industries, American Defense MFG, etc.) there are some brands that hold their own unique tolerance that may not be directly cross-compatible with our barrels.
Bolt Coatings & Finishes
While many manufacturers will undersize their bolts to accommodate the additional material thickness of a coating or layer of additional surface material, some manufacturers may simply apply the coating to a normally sized bolt, leading to potential tolerance stacking and headspace issues.
Variance in Barrel Tolerances
While multiple layers of quality control and inspection processes are incorporated into the production of Criterion barrels, other barrel manufacturers may not be as concerned with holding tolerance as precise as those held by Criterion. A barrel may improperly headspace if a chamber is reamed too deeply or too shallow, if the shoulder of the barrel is improperly set or timed, if the barrel extension is improperly sized, or if the barrel extension comes loose after the salt bath nitriding process (an affliction normally encountered with improperly treated budget barrels).
What is a Short Chambered Barrel?
Many Criterion barrel models are sold with an intentionally short chamber. This allows the gunsmith a certain level of flexibility in finish reaming the barrel to accommodate bolt dimensions of varying size. This feature is most commonly found in our vintage military rifle barrels, ranging from the Krag Jorgensen to the M14.
Normally each of these barrels is short chambered by .010. With most short-chambered barrel models this means additional material will need to be removed from the chamber to allow the barrel to headspace properly.
With rimmed cartridges (.30-40 Krag, .303 British, etc.) Criterion has added an elevated rim around the base of the breech face where the breech face engages with the case rim. Gunsmiths will typically need to remove some of this material to ensure that the chamber is properly headspaced.
It is worth noting that while most Criterion vintage military replacement barrels will need to be finish reamed, select models are sold with a finished chamber. This typically includes all chrome lined barrel models. Any attempt to ream out a chrome lined barrel post-production will serve to damage the chamber finish and is not recommended.
Due to the large variance in bolt tolerances typically found in vintage military rifles, the armorer may want to have multiple bolts on hand when attempting to install a chrome lined replacement barrel. If the original bolt will not headspace properly with a new replacement barrel (most likely due to excessive wear to the bolt lugs), then a second bolt will likely prove to be serviceable.
If you have any questions regarding the tooling requirements and process of checking headspace with each unique rifle design, feel free to give us a call at (262) 628-8749 or via email at firstname.lastname@example.org. We would be more than happy to walk you through each step of the process to help get your rifle build up and running!