Re: Hand Matching AR Bolt to AR Barrel - Answer this..
The current military specifications for the M16 series call for headspace of not less than 1.4646” and not more than 1.4706” on a new rifle. The commercial Forster/SAAMI gauges are marked in 1,000ths of an inch instead the 10,000ths of an inch that the dedicated M16 gauges are. When installing a new barrel, the commercial gauge measuring 1.465" can be used in place of the M16 "Go" gauge. Consider the 4/10,000ths of an inch difference an extra margin of safety. The commercial 1.470" gauge (the commercial "Field" gauge) can be used to confirm that the headspace is within spec for a new rifle. Unfortunately, there are no gauges available to measure between 1.470" and 1.4730", the latter being the measurement on the M16 "Field" gauge.
The M16 Field Service gauge measures a dimension of greater than 1.4730”. A rifle with a bolt that does not close on a Field Service gauge is considered safe to fire by the army, but not the Marines who use the military "No-Go" gauge for that measurement. Without using military Go or No-Go gauges, or commercial gauges marked with their measurements, you cannot know whether or not it is truly in spec. An interesting tidbit of information is that the Colt M16/AR15 Field gauge measures a dimension in excess of 1.4736”. It seems that the military has built in a bit more of a safety factor with their gauge.
You should remove the extractor parts and the ejector, which normally requires four hands. Removing the ejector is a two-handed job if you have the proper tools, specifically a Sinclair Bolt Vice for the AR15.
You can’t really reset the headspace on an AR15 with a chrome-lined chamber. That is set when the barrel extension is installed on the barrel. If you have an unlined barrel, with short headspace, a gunsmith can adjust it by cutting the chamber deeper with the appropriate chambering reamers. If the headspace is long, or the chamber chrome-lined, the only option is to try a different bolt until headspace checks good, or you run out of bolts. Then it is time for a new barrel.
To recap, the specs are as follows:
New Rifle Headspace: 1.4646" to 1.4706"
SAMMI headspace gauges to use: 1.465" and 1.470"
Unsafe Rifle Headspace: 1.4736"
Use the Colt M16/AR15 Field Gauge: 1.4736" or,
The US military gauge: 1.4730”
Note: The Forster/SAAMI "No-Go" gauge measures 1.467"
