There is some dispute over the need to check the headspace on AR15/M16 rifles on these forums. Some say you must. Some say you should. Some say you don't have to.
Whether you check the headspace on your rifle is up to you. The following is provided for your information…
First, try this URL for an explanation of Headspacing: [url]www.hybrid.ualr.edu/satu/headspace.html[/url]
Then try this one for more enlightenment: [url]www.fulton-armory.com/headspace.htm[/url]
The Forster/SAAMI gauges available from Brownells are primarily intended for use with .223 Remington civilian rifles, but can be used with your AR15. The M16/AR15 series use their own peculiar specs for headspace, making it necessary to ignore the "Go' and "No-Go" markings, and look instead at the measurement marked on the gauge.
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"
Sources for Gauges -
Brownells: [url]www.Brownells.com[/url] SAAMI gauges and a whole lot of new AR15 parts and accessories.
Sarco: [url]www.sarcoinc.com[/url] Military “Field” gauge as well as other maintenance items and M16/AR15 parts and accessories.