I have a 18" Mega Arms 5.56 barrel on a Midwest PX forged upper. It had several test rounds fired from it by the vendor. For giggles, I checked out headspace with my Clymer go/no go gages. The AIM Surplus bolt would not close on a Clymer Go gage. I swapped it out for my BCM BCG and it still would not close.
From prior searches here I have seen recommendations to simply use the field gage because that is what the TCM says to do. I was under the impression that just let me know when I was at maximum headspace on a worn rifle before it became dangerous.
Anyway, a bolt not closing on a GO gage indicates a short chamber. That seems like a bad idea on a semi-auto rifle. Any recommendations on how I should proceed?
If a chamber is .001" short it will refuse a gauge but not make any difference in how it will shoot especially if you reload and adjust the size die to the chamber. It’s obviously not too short because it chambers the ammo. As you shoot it more the bolt and extension lugs will mate and the headspace will grow a little. The go may fit after 50 shots.
Factory ammo is headspaced a few thousandths below SAAMI min. Ammo manufacturers do this so their ammo will cycle 100% in all rifles. If you reload, you can size your brass to each specific chamber, even if they are fairly far out of spec.
This is why your rifle failed the test but will still cycle factory ammo just fine. Your rifle has a short headspace but is fine how it is. You can work around a short headspace (within reason). A longer headspace is what is dangerous.
Did you remove the ejector and extractor prior to gauging? was the chamber clean? Army uses a single field gauge, thats cut to accomodate the extracor/ejector. My COLT/Brownells gauge instructions to strip the bolt. Also, were you using the 5.56mm Go gauge (GO556NATO) or the .223 gauge (G0223REM) ? Not looking to sharp shoot you or any thing. I’ve also heard competion guns sometimes have tighter chambers. They have/had gauge sets that let you measure the headspace exactly.
I’d be more concerned about it failing the no-go or field. Headspace grows with use and wear. Just don’t put any M855A1 through it, that stuff is loaded HOT!
What cd228 said. Did you strip the bolt first before trying it with the go/no go gauges?
Hi, I did the second time around after having read CD228’s post. It still failed the Clymer 5.56 Go gage.
Hmm, i’d try a field gauge just for safety’s sake. I’d also recomend reaching out to the barrell maker.
If it doesn’t close on a go gauge, it certainly isn’t going to close on a field gauge. What exactly will this resolve?
You are assuming that his go gauge is properly marked and sized. If the go gauge is properly marked , you are right and running the field gauge will tell us nada. Also, if he calls the maker and the maker tell him that they only use field gauges, the OP can say “it failed the field gauge too”. Plus, to be honest, I’m Army and we only use the field gauge for headspace (we just did our arms room) so I just typed that in on auto pilot.
What course of action do you recommend?
I would send the barrel, bolt, and gauge to the manufacturer.
Not a bad idea, I’d still contact them first.
Yes, I did assume a phone call. I’m just thinking that while possible, it isn’t probably that a clymer gauge is bad. Still, allowing for that possibility, I would send it in for the manufacturer to be able to replicate the problem.
Definitely a good plan. I agree that the gauge being bad is low probability. Where did that OP get to?
I am still here. Work internet filters and a one and a half year old daughter conspire against me visiting more often. I spoke with the vendor who sold me the upper and am feeling better about it. The chamber is a little bit short, but the bolt does fully close on sized dummy rounds. I do have the option of sending the upper back, but am leaning towards keeping it.
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How is the little one doing. We are running throught flu season here and a number of my co-workers’ kids have gotten sick. As for the upper, as I mentioned before some guns do run tight. I’d just pay closes attention to safety and safety equipment. I stil recomend the above course of action Frac and I outlined above. I freely admit I tend toward the cautious side when It comes to other folk’s gun and gear.
We are all doing well around here. Fortunate not to get the colds and stomach viruses that are making the rounds in the kiddie circles.
I figured I would include my correspondence with the vendor, in case anyone was interested in what he had to say. I X’d out his name for anonymity sake.
Hi XXXX,
We have been fully snowed in for quite a while up here in New Hampshire, so I haven’t had a chance to shoot the upper I bought. Since I bought an AIM Surplus BCG, I figured I would check the headspace. I removed the extractor spring and o-ring, and could not close the bolt on the no-go, which is good. But I couldn’t close the bolt on the Go gage either, which is bad. I swapped out the new BCG and used the one from my Bravo A4 rifle and got the same result. I used both gages on the BCM rifle and they worked correctly (i.e. bolt closed on Go gage and didn’t on no-go). I can close the bolt on 223 snap caps and a live 5.56 round in the upper I bought from you, but am worried about the chamber being cut short. If I end up shooting a round that i sized just a little too big, I could potentially have the bolt not fully lock up and the rifle could fire out of battery. I figured I would contact you and see if you had any ideas.
Thanks,
Chris
Hey Chris,
I wouldn’t sweat it. OOB detonations with ARs are more or less impossible due to the design of the bolt lugs and the firing pin. If the lugs aren’t engaged, the hammer can’t strike the head of the firing pin. Using gages can be a mixed bag too. Having worked with a few barrel and bolt manufacturers, I’ve learned that gages are really only useful when you match the gage with the reamer as gages are built for reamers, not calibers. For instance, the chambers of my house brand midlength barrels are cut with a PTG reamer so I use a PTG gage to check them. I can use other brands of gages but they can sometimes throw a false positive/negative. Your barrel has a match chamber so it should be on the tight side making it will be even more sensitive to gage/reamer matching than a non-match chamber/barrel. This is another reason why I believe in test firing uppers before delivering them. If there was an issue with the chamber being cut short, there would have been some signs in the firing cycle and in the spent brass. I would headspace your reloads to duplicate FGMM (which in my experience are on the short side to reliably load in anything) initially and then just neck size your fired brass, knocking the shoulder back by .001 or .002. Fun fact but I’ve measured before and after with .308 FGMM and the headspace difference varied between .01-.015 in three barrels, two of them being match grade.
If it worries/bothers you, send the upper back and I can do a more thorough test fire with a variety of ammo with before and after headspace data for you.
XXXX