Ask Ian: What is Headspace? (And Why It Matters)

From LongBeef on Patreon:
“What exactly is headspace? And how important is headspace in old milsurp guns vs. more modern guns?”

Headspace is basically the amount of play a cartridge has in a chamber. There has to be some to account for manufacturing variations in guns and ammunition, but too much or too little can cause problems. Too little can cause extraction problems and poor accuracy. Too much can cause poor accuracy as well, and also shoulder separations, case head separations, and kabooms.

Here’s a neat prototype rifle that operates based on headspace:

Also shown in this video:
Ian’s Terrible Krinkov:
Turkish “Enfauser”:


  1. “(…)how important is headspace(…)”
    This greatly depend on material of case, consider history of multi-ball 12,7×108 cartridge
    which was developed in Soviet Union, they made brass version and steel version and when they start to test it, latter was found to be somewhat capricious, it did not work properly in certain types of 12,7 mm machine gun, even despite all were supposedly using very same 12,7×108 mm cartridge. Further examination showed that chamber dimensions differ slightly between (DShKM), (NSV-12,7 and A-12,7), (9-A-624 which would later be known as YakB-12,7), see 6th image from top. This was not problem for elastic brass, but steel was not elastic enough to adjust for that. As mentioned machine gun were already accepted, which meant chambers dimensions were etched in stone, it was decided to put solely brass cases into production.

    • This is most unfortunate, from production standpoint, seeing how much of a brass could be saved in this big case, of you were to switch to steel.
      I wonder is it possible to make some kind of brass/steel laminate in cases (if that would work, anyway)?
      Like inner or outer shell being different metals, thus conserving at least 30% or 50 of the more expensive one.

  2. Very well done. I am locally known as somewhat of a gun “expert” and have had to attempt to explain headspace to several people. I am saving this video so I can verify my concerns to the gun owners in the future. Thanks for explaining a difficult subject.

    • So in the example of ideal headspace what in general is the measurement of said space? Or does it vary widely per firearm?

      • Varies more by the cartridge/action interface than anything else. Headspace on a slam-fire mechanism isn’t all that critical, especially in a pistol-caliber weapon.

        Hell, I’ve seen blowback weapon brass that was so badly bulged around the case base that it wasn’t even funny, and the weapons kept right on working just fine. Those were M3 submachineguns near the end of their service lives, but, still…

  3. “(…)What exactly is headspace?(…)”
    Wait, it is headspace XOR head space?
    TM 9-226


    available at gives following succinct definition
    Head space.—The head space of a military weapon with a
    cartridge fully seated in the chamber is the distance between the base
    of the cartridge and the face of the bolt.

    • Umm, no.

      The correct definition of headspace is the full distance from whatever the seating limit of the cartridge is to the bolt face.

      In a rimmed or semi-rimmed cartridge, this is the thickness of the rim. Examples; .38 Special, .30-30 Winchester, 7.62 x 54Rmm Mosin-Nagant, .38 Super Automatic (9 x 23SRmm), 6.5 x 50SR mm Arisaka.

      In a belted cartridge, it is the distance from the front edge of the belt to the bolt face. Examples; .300 Winchester Magnum (7.62 x 67Bmm), 7mm Remington Magnum (7 x 64Bmm), .375 Holland & Holland Magnum (9.5 x 73Bmm).

      In a rimless cartridge, it can be;

      1. The distance from the bolt face to the case mouth, which is typical for “straight-walled” cases such as most automatic pistol cartridges; examples include the 9 x 19mm Parabellum/Luger/NATO, 9mm Bergmann-Bayard Long/Largo (9 x 23mm) and the .45 ACP (11.43 x 23mm). The .30 USC (7.62 x 33mm) is an example of a rifle cartridge in this category (one of very few, in fact).


      2. In a bottle-necked cartridge, the distance from the case-head/bolt face to the shoulder of the case; examples- .50 BMG (12.7 x 99mm), 14.5 x 114mm, and most modern rifle cartridges. Also some pistol cartridges such as 7.63 x 25mm Mauser, 7.62 x 35mm Tokarev, .357 SiG/Auto (9 x 22mm), and the 5.45 x 18mm PSM.

      The latter category can sometimes cause problems. Note that contrary to popular belief, “.30 Mauser” and “7.62 Tokarev” are not the same cartridge and are not safely interchangeable. The Tokarev case shoulder is further forward and squarer than the Mauser’s. A Mauser cartridge in a Tokarev chamber will usually just “fire-form” to Tokarev case profile; a Tokarev cartridge in a C/96 Mauser has a good chance of causing a breech failure due to excessive pressure.

      This is even more notable with some rifle cartridges. Such as the supposed “interchangeability” of the .500 Jeffrey and 12.7 x 70mm Schuler big-game cartridges. No, they are absolutely NOT interchangeable. Don’t even try it.

      The Jeffrey case has a longer and shallower shoulder angle than the Schuler. In a Schuler chamber, the Jeffrey case will “seat” too far forward, creating excessive headspace.

      The Schuler’s shoulder is shorter, squarer, and more sharply angled than the Jeffrey’s, and further up the case toward the case mouth. It can be forced into a Jeffrey chamber, and this creates a “crush fit”- with the attendant problems of excessive pressure.

      Either one is extremely dangerous with the working pressures of these two high-intensity big-game loads.

      So although they look a good bit alike, do not get the wrong one in the wrong rifle. You won’t like the results.

      Headspace is measured in different ways depending on cartridge and chamber design. Using the wrong method for the arm in question can have nasty consequences.

      clear ether


      • So there is head space and headspace which have vastly different meanings. I did not take adjustment for chaos-worship nature of English language.

        • Keep in mind that when headspace is properly adjusted, the distance between the case head and the bolt face should be effectively ZERO.

          In fact, using brass shims of known thickness between the two while closing and locking the bolt is the classic custom gunsmith’s method of determining if excessive headspace exists, and if so how excessive it is.

          And yes, I know the Tokarev is 7.62 x 25mm, not 7.62 x 35mm. Finger malfunction.



      • There’s a very interesting one with the 12.7 x 70 Schüler and the imaginary. 500 Jeffery that Kynamco came up with…

        I don’t think that anyone can show that any of the surviving original .500 Jeffery rifles nor any of later rifles up to the CIP adoption of the kynamco dimensions, are chambered for the case that kynamco came up with

        There were only something like 28 rifles in the original series of Jeffery built “.500’s” (12.7x70mm Schüler) rifles,

        The Link relates the story of two examples of those original 28 Jeffery rifles, “failing” to take the “go” gauge for the kynamco “.500 Jeffery” at the CIP Birmingham Proof House

  4. Man, Couple that Turkish “Enfield” Frankengun with Turkish surplus ammo, which has a reputation for being hot…Well, how much do you like your face?

  5. I am told that the longevity of rimmed cartridges in military service is due to the relative ease of measuring and then manufacturing headspace from the chamber mouth to the bolt head; for rimless cartridges this has to be measured from the shoulder in the chamber for bottlenecked cartridges or from the step for the mouth of a straight-walled or tapered cartridge. In British service the manufacturing convenience outweighed any consideration of rim jam.

  6. “Check your Headspace”: something one should do regularly and in all contexts.
    Great presentation (no need for a graphics department yet). Great topic. Not for the geeks maybe, but there are always newcomers to the community and Ian had done them a service with this post.

  7. OK Ian, you may have had someone at somepoint re-cut your chamber to .257 Roberts.
    Have run across a few of these. FWIW.

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