AR-15 Headspace: What It Is, How to Check It, and Why It Matters
What This Article Covers
This article covers what headspace is in an AR-15, how to verify it with go and no-go gauges, when the check is required, and what the consequences of incorrect headspace are. For context on the barrel and bolt components that determine headspace, see the barrel guide and the bolt carrier group guide.
Key takeaways
- Headspace is the distance from the bolt face to the datum line of the chamber.
- In an AR-15, headspace is set by the combination of bolt and barrel, not either part alone.
- A go gauge confirms the chamber is deep enough; a no-go gauge confirms it isn't too deep.
- Headspace must be verified any time a new bolt is paired with a new barrel.
What Headspace Measures
Headspace is the distance from the bolt face to a specific reference point inside the chamber (the datum line) where the cartridge case shoulder stops when fully chambered.When a round is chambered, the case sits against the datum. The bolt closes behind it, and the locking lugs engage the barrel extension. Headspace is the measurement of that relationship between the closed bolt face and the datum.
In a correctly headspaced rifle, the cartridge case fits snugly with minimal clearance. When the round fires, the case expands to fill the chamber, seals the chamber against gas, then contracts slightly as pressure drops, allowing the extractor to pull it free.
Why Incorrect Headspace Is a Problem
Headspace that is too tight means the bolt cannot fully close on a live round, or can close but crushes the case shoulder. The result is a rifle that won’t chamber standard ammunition.
Headspace that is too long means the case has to stretch to fill the gap between where the case shoulder sits and where the bolt face is. Each firing stretches the brass further. Severe excess headspace causes case head separation : the base of the case separates from the body during firing, releasing pressurized gas into the action. This is a dangerous failure.
The practical concern is excess headspace, not insufficient. Most factory-new components that fail headspace checks are too long, not too short.
The AR-15 Bolt System and Headspace
The AR-15 uses a rotating bolt that locks into a barrel extension (the steel collar at the rear of the barrel that accepts the bolt’s locking lugs). Headspace is determined by the geometry of the bolt’s locking lugs and the locking lug recesses in the barrel extension.
This means headspace is a property of the bolt-and-barrel combination, not either part individually. A barrel can be correctly manufactured. A bolt can be correctly manufactured. Their combination may still produce headspace that is out of spec if they fall toward opposite ends of their respective tolerance windows.
This is why headspace must be checked with the actual bolt that will be used in the actual barrel, not assumed based on the quality of each part in isolation. For example, pairing a BCM 16” mid-length barrel with a BCM M16 BCG from the same manufacturer still requires verification. BCM tested their barrel with their own test bolt, not the specific BCG you purchased.
The Three Gauges
Headspace gauges are precision-ground cylindrical tools that simulate cartridge dimensions. They are caliber-specific and not interchangeable between chambers.
Go gauge: Represents the minimum acceptable headspace. When inserted into the chamber, the bolt must close completely on the go gauge. Failure to close means the chamber is cut too shallow. This is a barrel defect and is not adjustable in the field.
No-go gauge: Represents the maximum acceptable headspace. The bolt must not close on the no-go gauge. If the bolt closes freely on the no-go gauge, headspace is excessive and the rifle should not be fired until the issue is resolved.
Field gauge: Represents the absolute condemnation limit. If the bolt closes on a field gauge, the rifle is unsafe to fire and must be removed from service. Field gauges are used by armorers to assess rifles that have been in extended service. Most home armorer work requires only go and no-go gauges.
Caliber-Specific Gauges
Gauges must match the specific chamber. Common examples:
| Chamber | Gauge Needed |
|---|---|
| 5.56 NATO | 5.56 NATO gauge |
| .223 Remington | .223 Remington gauge |
| .223 Wylde | .223 Wylde gauge |
| 6mm ARC | 6mm ARC gauge |
| .308 Win / 7.62 NATO | .308 Win or 7.62 NATO gauge (different specs) |
| 6.5 Creedmoor | 6.5 Creedmoor gauge |
5.56 NATO and .223 Remington have different maximum pressure specifications and slightly different chamber dimensions. A rifle chambered in 5.56 NATO should be gauged with a 5.56 NATO gauge. Using a .223 gauge on a 5.56 NATO chamber, or vice versa, produces an incorrect result.
When to Check Headspace
Any new barrel paired with any new bolt. This is the primary scenario. The manufacturer checked their components with their own test bolt; your combination may fall outside spec even if both components are quality parts.
Any surplus or second-hand bolt. Used bolts have unknown service history and may be at the edge of their tolerance window.
After any barrel work. Re-chambering, set-back operations on worn throats, or any modification that changes the chamber geometry requires a fresh check.
When extraction problems appear with case deformation. If fired brass shows a bright stress ring near the base (indicating the case is stretching), headspace warrants measurement. See the malfunction diagnosis guide for how to read brass condition.
How to Check Headspace
The gauge check itself is bench work with no live ammunition. No hearing protection is required for gauge verification. If you proceed to a live function check after confirming headspace, wear ballistic-rated eye protection (ANSI Z87.1) and hearing protection (NRR 29+ earplugs or NRR 22+ earmuffs) before firing.
- Remove the bolt from the carrier. Drift out the cam pin, remove the firing pin, and rotate the bolt out of the carrier.
- Insert the go gauge into the chamber (from the muzzle, or hand-fed with the upper tilted).
- Insert the bolt into the barrel extension by hand (no carrier needed).
- Rotate the bolt to the locked position. It must close completely. Resistance that prevents full lock-up indicates debris in the chamber; clean and retry before concluding the chamber is undersized.
- Remove the go gauge. Insert the no-go gauge.
- Attempt to rotate the bolt to the locked position. It should stop. The bolt should not fully lock on the no-go gauge. If it locks fully, the headspace is excessive. Do not fire the rifle.
If the bolt closes on the no-go gauge, the barrel, bolt, or both are out of spec. Determine which by testing the bolt in a known-good barrel (or the barrel with a known-good bolt). Replace the out-of-spec component.
Maintaining Headspace Over Time
Headspace does not typically change in a well-maintained rifle under normal use. It can shift under the following conditions:
- Throat erosion: High round count, particularly with hot loads, can erode the throat (the area just ahead of the chamber). This does not directly change headspace but is a sign of a barrel that may be approaching end of service life.
- Locking lug wear: Sustained high-round-count use can cause wear on the bolt’s locking lugs or the barrel extension recesses. A bolt that previously passed a headspace check can eventually fail one.
- Mixing components after separate purchases: If you replace a bolt on a rifle that has an existing barrel, the new combination must be checked even if the original combination was verified.
Periodic headspace checks (at high round counts, after any component swap, or if extraction problems develop) are part of responsible rifle maintenance.