What Is AR-15 Headspacing and Why Does It Matter?
Headspacing is the distance from the bolt face to the datum point in the chamber where the cartridge stops. Correct headspacing ensures the cartridge is held at the proper position — too much headspace can cause case ruptures and gas blowback, while too little headspace prevents the bolt from closing fully. On the AR-15, headspacing is controlled by the barrel extension and the bolt, not the barrel itself.
Why It Matters
Incorrect headspacing is a safety issue. Excessive headspace allows the cartridge case to stretch beyond its elastic limit, potentially rupturing and sending hot gas into the receiver and toward the shooter. Insufficient headspace prevents the rifle from going into battery. While factory barrels and bolts are almost always within spec, checking headspace is critical when mixing bolt carrier groups and barrels from different manufacturers or using reloaded ammunition.
The Detail
How AR-15 headspacing works:
- The 5.56/.223 cartridge headspaces on the shoulder datum (the angled portion of the case)
- The bolt locks into the barrel extension with 8 locking lugs
- When the bolt is fully locked, the distance from the bolt face to the shoulder datum in the chamber must fall within specification
- GO gauge: bolt should close on this gauge (minimum chamber dimension)
- NO-GO gauge: bolt should NOT close on this gauge (maximum safe dimension)
- FIELD gauge: bolt closing on this indicates unsafe, excessive headspace — do not fire
Specifications:
- 5.56 NATO / .223 Rem GO: 1.4636 inches
- 5.56 NATO / .223 Rem NO-GO: 1.4666 inches
- 5.56 NATO / .223 Rem FIELD: 1.4696 inches
- Maximum tolerance from GO to NO-GO: 0.003 inches
When to check headspace:
- When assembling a new upper from separate barrel and bolt
- When replacing a barrel on an existing upper
- When using a bolt from a different manufacturer than the barrel
- When buying a used rifle with unknown history
- When experiencing case head separation or difficult extraction
- NOT necessary when buying a complete factory upper (already checked)
How to check headspace:
- Remove the upper from the lower
- Remove the firing pin and extractor from the bolt
- Insert a GO gauge into the chamber
- Close the bolt on the GO gauge — it should close fully
- Insert a NO-GO gauge into the chamber
- Close the bolt on the NO-GO gauge — it should NOT close fully
- If the bolt closes on the NO-GO gauge, check with a FIELD gauge
- If the bolt closes on a FIELD gauge — do not fire, replace bolt or barrel
Gauge costs:
- Individual gauges: $25 to $40 each
- GO/NO-GO set: $50 to $70
- Brands: Clymer, Pacific Tool & Gauge, Forster
- Worth owning if you build multiple rifles or swap bolts/barrels frequently
Common headspace issues:
- Bolt closes on NO-GO but not FIELD: technically out of spec but generally safe with factory ammo; monitor for signs of excessive headspace
- Bolt does not close on GO gauge: bolt and barrel extension fit too tight; try a different bolt or check for debris
- Case head separations during firing: strong indicator of excessive headspace — stop firing immediately and check
Build Impact
If you buy a complete upper assembly from a reputable manufacturer (BCM, Aero, DD), headspacing has already been checked and you do not need to recheck it. If you assemble your own upper by buying a separate barrel and bolt carrier group, invest $50 to $70 in a GO/NO-GO gauge set and check before firing. It takes 2 minutes and ensures safe operation. When in doubt, take the upper to a gunsmith — a headspace check costs $15 to $25.