Strange Problem with a Select-Fire AR

SiersBy John Siers a.k.a. “Gunsmith Jack”

I love ARs because they bring me so much business. On the other hand, I hate ARs because everyone who owns one thinks he’s a gunsmith. Can’t begin to tell you how much time I’ve spent (and money I’ve made) fixing really silly problems created by weekend warriors trying to “tweak” or “improve” or just “dress up” their ARs. Fortunately, none of these people have gotten hurt… yet.

Got a call the other day from a local law enforcement agency (yes, I’m an AGI-Certified Law Enforcement Armorer) and they wanted me to take a look at a select-fire AR that had behaved very strangely at the range. It had fired just fine in semi-auto mode, had in fact gone through several magazines without a hitch. Then came time for the full-auto qualifications, and the gun fired one round… then quit cold. Trigger wouldn’t reset, manual cycling had no effect. Gun simply would not fire, even when put back in semi-auto mode.

The good news was that they could cycle it manually – charging handle worked just fine, bolt came back with no problem. In other words, they were able to empty and safe the gun with no problem; but that trigger still wouldn’t budge in either semiauto or full auto mode. So… they called me and I went down to the police station the next morning to look at it.

The gun was a genuine U.S. Military M-16A1 on loan to the local police department through a surplus equipment program by which law enforcement agencies can get military grade equipment from the Federal Government. My first thought was to examine all the goodies inside the lower. Hammer was cocked when I opened the gun, being held by the full auto latch at the top of the lower. I didn’t think much about that, just released it and went on to verify that everything inside was in perfect working order. Secondary sear engaged properly in semi mode, and released the hammer to the primary sear when the trigger was released. With full auto selected, the secondary sear disengaged and the full auto latch grabbed the hammer as it should. It also released the hammer properly when the latch was tripped forward. OK… let’s put the gun back together so we can see how everything works when we put it through its paces while manually cycling the bolt.

Semi auto worked fine. Full auto… Hmmm. OK, here’s the problem. Bolt carrier comes forward, but the full auto latch doesn’t trip so hammer stays locked back, a captive of the latch… and of course the trigger won’t reset under those conditions. In fact, if the latch doesn’t get tripped, that hammer isn’t going anywhere – gun is totally disabled.

OK, take it back apart again and pull the bolt carrier group. Well… DUH… now the problem is obvious. Some gunsmith wannabe police officer (who shall remain nameless… the Chief knows who he is and is probably roasting him over the coals as I write this) had been taking guns apart. He took this one apart, and just for comparison he also took apart his own personal (semiauto only) AR. Then he put them back together… but he got the bolt carrier groups mixed up, and his own semiauto bolt ended up in the select fire AR. Here’s a photo of the two groups for comparison and you can see the problem.

The one on top is the select-fire bolt carrier group. The one on the bottom is the semiauto group that was in the gun. Of course, the bottom lip on the rear is what’s supposed to trip the latch as the bolt carrier comes forward. In this case, the lip on the semiauto carrier is too far to the rear. Even with the bolt locked up, it doesn’t come far enough forward to even touch the latch.

Of course, Officer Gunsmith never noticed the problem. His civilian AR worked just fine with the select fire bolt carrier; but if one of his fellow officers had gotten into a situation where he needed that M16 to work in full auto…

In other words, I’m glad they found the problem on the range instead of on the street. I’m also glad I was able to fix it for them.


4 Responses to Strange Problem with a Select-Fire AR

  1. SHEESH! One of those painfully obvious kind of things! My colt HBAR has a BCG like the one in your bottom photo except the lip is even smaller if you can imagine! And there’s also a slot cut in it! they really went out of their way to make sure that one would never work in an auto set up! the rest of my AR’s wear M16 BCG’s. Excellent article sir! Reiterates the point that if you don’t know how it all works you ought not mess with it.

  2. Yes, I love the AR-15 platform for the reason stated–it brings me a lot of business. Just a few days ago a young man brought me his beautiful (and expensive) AR he put together himself. Only problem was it was an expensive single shot.

    He never tightened the set screws for his low-profile gas block, and he installed the gas tube upside down. Needless to say, it was getting no gas into the BCG.

  3. The only stuff I do to my guns is field strip and clean. If something doesn’t ‘feel’ right I head directly to the range where there are a few seasoned gunsmiths. I did however change the charging handle on my Sig ar15 and to play it save I had a video at the ready. The same video I have at the ready when field strip and clean. I envy those that buy all the ar parts/kits and put the gun together and it actually goes bang.

    • Very smart of you to play it safe! Give it some time and practice who knows maybe you’ll be that seasoned gunsmith some day. You’d be surprised how many people can’t even field strip and clean one.

      I got an AR carbine from a local deputy who couldn’t get it back together. Someone had unscrewed the buffer tube which allowed the buffer retaining pin to depart controlled flight. I don’t even want to know the full story as the rifle was brought to me in pieces. A new retainer spring a little time (and embarrassment on their part) and she was good as new.

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