Headspace determines whether an AR-15 chamber and bolt fit safely with a given cartridge. Here's what it means, when to check, and how. This guide breaks the topic down in plain language so you can make a confident decision for your build.
Headspace is one of those AR-15 topics that sounds intimidating but is actually straightforward. If you buy a complete upper from a reputable manufacturer, headspace is already set correctly and you don't need to do anything. If you're building your own upper or swapping components, checking headspace takes about 5 minutes with $80 of gauges. Here's what you actually need to know.
What Headspace Actually Means
Headspace is the distance from the face of the bolt (when in battery) to a specific datum point in the chamber where the cartridge is stopped. For rimless bottleneck cartridges like 5.56/.223, the datum is a specific point on the shoulder of the chamber.
When you close the bolt on a cartridge in the chamber:
- The cartridge is stopped by the chamber shoulder pressing on the cartridge shoulder
- The bolt face contacts the cartridge base
- The distance between these two points is the headspace
This distance has to fall within a small tolerance window: too tight and the bolt can't close; too loose and the case has room to stretch during firing.
Why Headspace Matters
Correct headspace: cartridge sits securely, bolt locks fully, case fires and extracts normally, brass lasts many reloads.
Too tight (bolt won't close on GO gauge): rifle won't chamber some ammunition; slightly out-of-spec brass jams; potential dangerous overpressure if you force the bolt closed.
Too loose (bolt closes on FIELD gauge): case can stretch during firing, causing case head separations (extractor rips off the case head, leaving the rest in the chamber). This is a malfunction that stops the rifle and can be a safety issue if repeated.
Headspace is not something to ignore when you're assembling from mixed components.
When You Need to Check
Always check:
- New barrel installed on any bolt
- New bolt used with an existing barrel
- Mixing a barrel and bolt that weren't shipped together
- After a bolt or bolt lug repair
- After several thousand rounds if accuracy or reliability degrades
Not necessary:
- Complete upper from a reputable manufacturer (BCM, Aero, DD, KAC, SOLGW, etc.)
- Complete rifle from a reputable manufacturer
- Bolt still in the barrel it was headspaced with at the factory
- Low round counts on a rifle you have documentation for
Headspace shouldn't change over time with normal use. It changes when metal moves: a lug shears, the bolt face erodes, or the chamber wears past spec. All of these are unusual.
GO, NO-GO, and FIELD Gauges
You need three gauges to fully check headspace:
GO gauge: Represents minimum acceptable headspace. Bolt MUST close on GO. This confirms the chamber is at least deep enough for the shortest in-spec ammunition.
NO-GO gauge: Represents maximum acceptable headspace for new commercial ammo. Bolt should NOT close on NO-GO. This confirms the chamber isn't longer than spec.
FIELD gauge: Represents the maximum acceptable for military surplus (looser tolerance). Bolt should NOT close on FIELD, even after many rounds. If a rifle passes GO but closes on FIELD, retire the barrel.
For a new AR-15 build with commercial ammo: bolt closes on GO, doesn't close on NO-GO. That's correctly headspaced.
For a used rifle you're inspecting: bolt closes on GO, doesn't close on FIELD. It's still safe to shoot with mil-surplus. Closing on FIELD means retire the rifle or replace the barrel.
How to Check Headspace
Tools needed: GO, NO-GO, and FIELD gauges for your caliber (5.56 for a 5.56 chamber; .223 for a .223 chamber).
Method:
- Unload the rifle completely. Remove the magazine, check the chamber, then check again.
- Remove the extractor and ejector from the bolt. These push on the gauge and give false readings. Most bolts require pin punches to remove.
- Clean the chamber and bolt face. Any fouling affects the reading.
- Insert the GO gauge in the chamber. Slowly close the bolt by pushing the carrier forward.
- Feel for lockup. The bolt should rotate fully into battery with no resistance. If it doesn't close, the chamber is too tight (or the gauge is dirty; reclean and retry).
- Extract the GO gauge and insert the NO-GO gauge. Slowly try to close the bolt.
- Bolt should NOT fully close. You should feel resistance and the bolt should not rotate fully into battery. If it closes with a NO-GO gauge, headspace is loose.
- Insert the FIELD gauge. Bolt should NOT close on FIELD gauge either.
- Reinstall the ejector and extractor.
The whole process takes 5 minutes on an assembled upper.
Interpreting Your Results
Bolt closes on GO, doesn't close on NO-GO: Correct headspace. Rifle is safe to shoot with commercial ammo. If you'll shoot mil-surplus, also check FIELD.
Bolt closes on GO, closes on NO-GO, doesn't close on FIELD: Headspace is slightly loose but within military tolerance. Safe with mil-surplus; commercial brass may have shorter life due to stretching.
Bolt closes on GO, closes on FIELD: Headspace is out of spec. Retire the barrel or replace the bolt (verify with a new bolt first before condemning the barrel).
Bolt won't close on GO: Chamber is too tight. Barrel needs to be reamed or the bolt lugs need to be checked. Do not force the bolt closed.
If you're not comfortable interpreting or acting on results, hand the assembled upper to a gunsmith. Headspace is one of those areas where "close enough" isn't good enough.
Browse pre-headspaced complete uppers and matched bolt/barrel combinations at ar15parts.com.
