Headspace is measured in different ways on different calibers. For a rimmed cartridge such as the 7.62x54R (where R stands for “rimmed”) headspace is the distance between the bolt face and the face of the chamber where the loaded cartridge rests. In a rimless cartridge such as a .308 the headspace is measured to the shoulder of the cartridge. This dimension is important for proper and safe firearm function regardless of the type of cartridge being checked. Too little headspace will not allow the bolt to fully close rendering the weapon unable to fire. Too much headspace can cause excess chamber pressure, as a result of the cartridge not being held firmly in the chamber on firing. This can be particularly dangerous given the chamber pressures generated by centerfire rifle cartridges..

Headspace gauges generally come in 3 sizes:

GO: measures the minimum acceptable headspace. This size is most often used when re-barreling or re-chambering a firearm.

NO-GO: This gauge is used to check for excessive headspace. If a firearm closes on the NO-GO gauge it is an indication that the weapon MAY not be safe to fire. Reloading ammunition for a firearm that fails NO-GO can result in unsafe loads due in part to the expended brass being elongated as a result of not being firmly chambered. Most military surplus firearms that pass NO-GO (the bolt will not fully close on the gauge) are considered safe to fire with surplus ammunition, or with modern loads not intended for reloading.

FIELD: A firearm failing the NO-GO spec can be tested on the slightly more forgiving FIELD gauge. Military chamber specifications are generally looser than commercial firearms, giving them a bit of tolerance for adverse conditions such as dirty chambers, weather extremes, etc. A firearm passing FIELD spec (not closing fully on the gauge) is generally considered safe to fire the ammunition it was designed to fire, i.e. military surplus ammunition of the designated caliber.

These are to be considered GENERAL GUIDELINES and are in no way intended to be an assurance of the safety of a particular firearm. If you have doubts about a firearm’s safety and usability you should have it checked by a professional gunsmith.

 

• Make sure the firearm to be checked is UNLOADED.
  
• Open and close the bolt on an empty chamber several times, making note of the amount of pressure needed to close the bolt. Knowing how the bolt feels on closing is important for proper usage of the gauge.
  
• Wipe the gauge with a clean dry cloth to remove the protective oil residue. These gauges are made from hardened and ground 4140 steel for precision and durability, but they WILL rust if not kept oiled when not in use. A drop of any light oil such as 3 in 1 or gun oil will be sufficient. Re-oil the gauge before storing it in it’s bag.
  
• Open the bolt far enough to allow you to insert the gauge on the bolt face, aligning the notch in the gauge with the extractor. The dimples in the gauge are for identification only and can face the chamber OR the bolt face.
  
• GENTLY slide the bolt forward and with light finger pressure close the bolt. If you feel resistance DO NOT FORCE THE BOLT CLOSED. This can damage both the gauge and the firearm being tested.
  
• The gauges are slightly magnetic and may stick to the chamber face when opening the bolt.
  
• For the GO gauge the bolt should FULLY CLOSE with minimal pressure to pass.
  
• For the NO-GO or FIELD the bolt SHOULD NOT fully close with minimal pressure to pass.
  
• Any firearm failing both the NO-GO and FIELD must be inspected by a qualified gunsmith before attempting to fire it.
  Failure to do so may result in personal injury or even death!

DISCLAIMER

Okie Headspace Gauges assumes no liability for injury or damage caused by unsafe or improper use, malfunctions, defects, or other factors related to the use of any firearm. Our gauges are intended for use only as a preliminary inspection tool, and as such are not intended to replace a full inspection by a qualified gunsmith.. Headspace is only one of a number of factors that can affect the safety of a firearm. Okie Headspace Gauges highly recommends that any firearm, regardless of age, be fully inspected by a gunsmith familiar with the type of firearm in question prior to use.