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Headspace is the dimension between the surfaces within the chamber of a firearm that hold the cartridge when the action is closed. If the headspace dimension is too small, a cartridge won’t chamber. If it’s too large, the cartridge will not be held securely during firing—backed-out primers and separated cases can be the result. Headspace Gauges are used to determine if a firearm’s headspace is within acceptable limits.
Headspace Gauges can be made to any dimension, but are generally made to the industry-standard (SAAMI) minimum and maximum dimension. On rimless, shouldered calibers (223, 308, 30-06), there is often an intermediate gauge.
- “GO” denotes the minimum headspace dimension on all headspace gauges.
- “NO GO” denotes the maximum headspace dimension on pistol, rimmed and belted gages. For rimless, shouldered gauges (223, 308, 30-06) it denotes the intermediate headspace dimension.
- “FIELD” denotes the maximum headspace dimension on rimless, shouldered gauges (223, etc.)
Headspace gauges are fit into the chamber of a gun like a cartridge would be. For the headspace of a firearm to be within spec., its action must close on the shortest gauge(“GO”), and not close on the longest gauge (“NO GO” for pistol, rimmed, or belted calibers; “FIELD” for rimless, shouldered like 223 or 308).
When checking headspace, ideally the bolt or slide should be stripped of anything that could cause a false reading (extractor, ejector plunger). Practically speaking, this isn’t always possible. Suggestions are given below for each type of firearm. Needless to say, these checks must be done on an UNLOADED gun, with a clean chamber and bolt.
With the bolt locked open, carefully fit the rim of the gauge under the extractor and center the gauge within the action–with plunger-type ejectors, you’ll have to counteract the spring pressure to hold it central. Make sure the plunger can be pushed below the bolt face—it may be too long, or jammed with crud and protrude to give a false gauge reading. Slowly close the action on the gauge while holding it centered within the chamber. The action should fully close on the “GO” gauge; it should not close on the “NO GO” or “FIELD” gauge.
Break-open Rifles or Shotguns: If you’re checking a break-open rifle or shotgun, the gauge is placed in the chamber and then the action slowly closed. DO NOT FORCE a break-open action closed. There is tremendous leverage here and your gun could be damaged by the hardened gauge if forced shut. The action should close on the “GO” gauge, but not close on the “NO GO” or “FIELD” gauge.
Semi-Auto Pistols: If possible, headspace is best checked with the barrel and slide off the frame and any recoil spring removed. Turn the slide upside down and fit the gauge under the extractor. Slide the barrel back toward the breech face and center the gauge within the chamber. Push the barrel toward the breech face while attempting to get the barrel to lock in its engagement surfaces. The barrel should lock up with the “GO” gauge in the chamber, but not lock on the “NO GO” or “FIELD” gauge.
If it’s not convenient to separate the barrel from the frame, lock the action open and slip the gauge under the extractor and continue as described above in the Rifles section.
These instructions apply only to locked-breech, not blow-back, semi-auto pistols.
Place the headspace gauge in a chamber as you would a cartridge and try to rotate the cylinder past the recoil shield. The “GO” gauge should rotate past the recoil shield; the “NO GO” and “FIELD” gauges should not.
These suggestions apply to most firearms. If you have questions or encounter problems gauging your particular gun, please call or email us.