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AR-15 Headspace Gauging: Go, No-Go, and Field

Procedure 8 min read

Professional AR-15 headspace gauging procedures using Go, No-Go, and Field gauges. Complete guide for working gunsmiths with safety protocols and troubleshooting.

Proper headspace verification is critical for AR-15 safety and performance. Incorrect headspace can result in dangerous pressure conditions, extraction failures, or premature case separation. This guide covers professional headspace gauging procedures using Go, No-Go, and Field gauges for the 5.56x45mm NATO/.223 Remington chamber.

Understanding AR-15 Headspace

Headspace in the AR-15 is measured from the bolt face to the datum line in the chamber—the point where the case shoulder contacts the chamber shoulder. For 5.56x45mm NATO and .223 Remington, headspace is measured to the 1.4636" datum line. The Go gauge represents the minimum acceptable headspace; the No-Go represents the maximum safe dimension; and the Field gauge is the maximum dimension before the rifle is considered unsafe to fire.

Gauge	5.56 NATO Dimension	.223 Remington Dimension	Result if Bolt Closes
Go	1.4636"	1.4636"	Normal — chamber at minimum
No-Go	1.4736"	1.4736"	Investigate — excessive headspace
Field	1.4836"	1.4836"	UNSAFE — remove from service immediately

A 5.56 NATO chamber has a longer leade than a .223 Remington chamber. Both use the same headspace gauges, but a rifle chambered in 5.56 will safely accept both cartridges. A rifle with a .223 chamber should not fire 5.56 NATO ammunition due to higher pressure in the shorter leade.

Gauging Procedure

Remove the upper from the lower receiver. Remove the charging handle and bolt carrier group. Separate the bolt from the bolt carrier. Insert the Go gauge into the chamber. Reinstall the bolt carrier with the bolt in the forward position and attempt to close the bolt by applying firm forward pressure. The bolt must close completely and rotate to the locked position on the Go gauge. If the bolt will not close on the Go gauge, the chamber is undersized—the barrel requires replacement.

Without removing the Go gauge, confirm the bolt closes and locks. Then remove the Go gauge and insert the No-Go gauge. Attempt to close and lock the bolt using only the weight of the bolt carrier—do not force it. The bolt must NOT close on the No-Go gauge. If it does, proceed to the Field gauge test.

Apply firm but not excessive pressure when gauging. Using the charging handle to force the bolt closed on a No-Go gauge can produce false results. Let bolt carrier weight alone determine whether the bolt closes. If you're uncertain, test three times and take the consistent result.

Field Gauge Protocol

Insert the Field gauge only after the bolt closes on the No-Go gauge. If the bolt closes on the Field gauge, the headspace is unsafe. Tag the rifle immediately as out-of-service and document the finding. Do not return the rifle to the customer until the barrel is replaced and headspace is re-verified. The root cause is typically a worn barrel extension, a worn bolt, or an improperly fitted aftermarket barrel.

A bolt that closes on No-Go but not on Field is in an acceptable but monitored condition. Document the finding, advise the customer of the condition, and re-gauge at the next service interval. Rifles used with steel-cased or bi-metal jacketed ammunition wear chambers faster and may benefit from more frequent headspace checks.

New Barrel Installation Verification

Always headspace-check after any barrel installation, barrel nut removal and reinstallation, or bolt replacement. Component combinations that individually measure within spec can produce out-of-spec headspace when combined. This is especially common with aftermarket barrels paired with military-surplus bolts that may be at the extreme of their tolerance range.

Headspace gauging takes under five minutes and prevents catastrophic failures. No AR-15 barrel swap is complete without it. If a customer brings in a rifle they built themselves and cannot produce headspace verification records, gauge it before it leaves your shop. A field gauge failure on a customer rifle is a liability event. Document every gauge test result.