It seems the "do I need to check headspace" question comes up once every couple weeks and although I knew the correct answer  (Check it, even the best manufacturers make mistakes too), I wasn't sure exactly what "headspace" was, how it was defined,  and what are the potential failure modes of a rifle with too much headspace.

So I did a little research and found a few things I would share that might help clear it up.   Hello Google.

First, a thousand words....no better yet, a picture.



An AR with too little headspace will not go into battery, (bolt closed, lugs engaged)   Because of the timing of the cam pin and cam track in the carrier, an AR that is out of battery should not fire.   The bolt should still be extended and the firing pin incapable of reaching the primer on the round.    However, the penalty for  a round firing out of battery is fairly severe I would think, so although many would say that a Field gage is all that is needed, I personally would prefer to see my bolt close nicely on a GO gage as well.

The bigger and more common issue is a rifle with too much headspace.   Initially I was thinking that too much headspace would only cause a case shoulder to bulge, split or rupture, possibly jamming the rifle and being just a general PITA.   However, then I read This
If you don't like a lot of reading just skip down to the Theory section.   It's the best explaination I've seen of case failure due to excessive headspace.

Best case if the case head separates from the rest of the brass, you get a lot of hot gases shooting down the mag well, and a stuck chunk of brass in the barrel.


Worst case, something like:



The cost of a set of headspace gages is nearly inconsequential to the amount of money most of us are spending on our builds.   The cost of a catastrophic failure is significantly higher.  

Some links to vendors of headspace gauges would be nice.  I haven't priced them, so I don't know if I'm getting ripped off or not.  



Nice info, too.

I got mine from PTG: http://shop.pacifictoolandgauge.com/index.php?main_page=index&cPath=160_161&sort=20a&page=1

https://shop.pacifictoolandgauge.com/

I would like to know where to get 5.56 headspace gauges with cutouts for the ejector.

The only reason I don't already have headspace gauges is because I don't know how to avoid .223 gauges listed as 5.56 gauges.

The two measurements in the red boxes is what contributes to a chambers headspace. If you subtract 0.156" from 1.6206" it will equal 1.4646". This is the minimum headspace (1.4646") listed in the military specifications for new barrels (all variants of the M16 and M4) that have been high pressure tested.

Actually an AR that will not pass a Go gage can go into battery.
Depending on how short the chamber is some ammo that is within SAAMI specs may not chamber.