From a recent thread, I think I should use a 1.4736" gauge to confirm safe headspace on a chrome-lined 5.56 AR. This gauge has been referred to as Colts Field II, the gauge Colt recommends for NATO, and Brownell’s part #319-418-033. (Thanks guys!)
How do I measure the gauge to verify it?
How do I prep for checking headspace, conduct the check, and interpret results?
Should I check when assembling and forget it? Check every thousand rounds? Every ten thousand?
Hmmmmmmm… Since I was the doofus that put my foot in it on that issue, I will attempt to pass on my “schooling”. Be aware that you are going to have several little “piles” of parts to keep track of. If you aren’t comfortable with having the bolt/carrier assembly completely stripped, find someone who is familiar with this procedure and have them do it.
Edit to add: Clean Clean Clean!!! chamber, gauge, and bolt (and carrier for that matter just to be sure no crud gets into the chamber during the test/measurement). Make sure the bolt face is spotless and that the locking lugs are spotless as well!!! This is the only way to insure the test is accurate.
Below is a condensed version of what Grant and later the BCM folks described:
Clear weapon (Safety first!!)
Break down weapon into its two major sub assemblies.
From the upper assembly, Remove Bolt carrier assembly and charging handle.
Remove bolt from carrier by removing the firing pin retaining cotter pin, firing pin, cam pin (from bolt at the “in” position in the carrier, rotate cam pin 90 degrees and withdraw), and finally pull out the bolt.
Strip bolt. (This must be done or the test is fubar.) This can get tricky if you haven’t done this before:
a) remove the extractor by compressing it against the bolt near the rear of the extractor to relieve spring tension and pushing the pin out with a small drift (up to 2.5 mm).
b) remove the ejector/spring by using a tool equivalent to Brownells P/N 939-000-003. After the ejector is compressed and secured, drift out the roll pin with hammer and small drift (1.5 mm).
c) I normally remove the gas rings, but I’ve seen folks successfully do this with the gas rings installed.
Now that the bolt is stripped, reassemble it into the carrier with the cam pin and firing pin to keep the cam pin straight. I always put the cotter pin back in to keep the firing pin in place. I do all this because you need the cam pin to locate and rotate the bolt and the cam pin must be kept aligned or the carrier assembly can jam in the closed position and you’ll have a b@tch of a time getting it out. (ask me how I know!!)
Insure the chamber is clean. This is critical to a proper reading.
Insert clean field gauge
Insert the carrier group into the upper and orient the upper assembly vertically (as shown in the thread you referred to). If the bolt does not close on the gauge, about 1/4 inch of the carrier will stick out the back of the upper. If this occurs the weapon is safe to fire, at least with Nato Spec Ammo. If the carrier goes into the upper flush with the rear of the upper, the weapon is not safe to fire and should be taken to a qualified gunsmith (or returned to the manufacturer for further review.
The procedure is simple for someone who is used to doing a complete strip of the weapon and detail strip of the bolt and reassembling it.
The Marine Corp manual also has an excellent description of how to do this. If your not familiar with stripping the bolt, I would consult the manual first.
Edit to add: I check headspace when I put it together. Actually I fit a bolt via a variation of the above process using my gauge set. Saami GO (1.4636), Saami NO GO (1.4666), Saami Field (1.4696) and now the Colt Field II gauge (1.4736). The issue that first got me into trouble on this was that I did not have the two larger gauges and did not realise that combat weapons are fit with more headspace to insure function in gritty grimmy dirty slimmy… well, you get the idea. Send me a PM if you want to pursue this offline. I rarely check headspace after initial assembly unless some “event” causes me to be suspicious.
woofe
Personally, I would only use a correct MILSPEC Field headspace gage. It’s what I use at work and what I use on my personal weapons. The TM does not call for a “GO” or “NO GO” or any other type of Field gage. Just my 700 Fils.
Yep, that is all you need to do. (Remember, I’m the anal retentive engineer.) I choose to determine, as close as I can, where the bolt/barrel assembly headspace starts at. Primarily I want to make sure that I don’t have the 1 in 10,000 bolt/barrel combination that has insufficient headspace (It could ruin my day). That is why I choose to use the other gauges.
Am I taking this too far…Well, yeah,… probably, but it keeps me off the streets and out of the bars.
But seriously, there are many folks who frequent this forum, who are real AR-15/M16 Armorers. From all indications they are very good at what they do. I build an occasional gun and enjoy doing so. I frequent this forum to learn from you folks and contribute where I can. Therefore, what I choose to do in addition to what is necessary, should not be taken as criticism of long established practice, but perhaps an exercise of my eccentricity. And where I’m just plain wrong, please, somebody point it out (Grant).
I understand from your location tab that you are in Iraq. Whatever your capacity that requires you to be there, thank you and get home to AZ safely to enjoy your new house.
woofe
woofe,
I am currently employed by a large PMC as an armorer in Iraq. I am currently enjoying my last few days of R/R here back home. Thanks for the kind words.