The Saga of How I Fixed My AR-9 SBR

I’m going to record my AR-9 trouble-shooting process in this blog post so that maybe someone else can glean some insight into all the things that can possibly go wrong, and how to fix them… because I lived through them.

I bought a DDLES “Glock mag” AR lower from Gunbroker once upon a time because that was the hotness like five years ago, and hard to get hold of. It was a smoking deal; based on the person’s other sales item and lack of engraving on the lower, I suspect they had an illegal SBR they were parting out in an attempt to make it go away. I dutifully filed a Form 1 on it, waited G-d only knows how long, and bought a cheapo PSA upper to put on it – I think it was 10.5″, whatever it took to get it across the magic 29″ OAL floor we have in MD.

It was a terrible performer right out the gate. It didn’t eject right. It didn’t cycle reliably. It tore cartridges in half. But, last night, after months of work, I got it working. How? Read on.

The first thing I did was tackle that whole “cartridges ripping in half”. I had assumed that my buffer was opening too early, so I went and bought a 308 Tubbs spring to put behind the heavy Spikes ST-9X buffer in the gun. This solved that problem, but my gun would no longer eject or cycle correctly. I would get short-strokes, and sometimes failures in ejection. (I verified the cycling issues by loading one in a mag and shooting it, to see if the bolt would lock back.)

Step 1 was to go back to a (new) carbine spring. No love. The cartridges were no longer ripping in half – maybe because the spring was new, or perhaps more likely, because I was no longer shooting horrible Freedom Munitions brass. But I still was not getting reliable ejection or cycling, even on relatively hot ammo like 9mm NATO. It was a little better, but not there.

Step 2: I called QC10 and bought a new ejector. I also shimmed it. This helped a little, too, but the problems still remained.

Step 3: I did a bit of research on the Internet, and realized that everyone screaming for a super heavy buffer was running a 16″ barrel gun. My 10.5″ might not need it. And, in fact, it did not. I bought one of the Foxtrot Mike “normal extended” 9mm buffers, and my gun was cycling properly again! Hooray! But ejection was still uneven.

Step 4: I bit the bullet and bought a new 9mm BCG. This was also a Foxtrot Mike product – it uses a “normal AR-style” extractor instead of the usual 1911-style extractor. All the sudden, my gun was ejecting reliably! MAGIC!

But now I noticed problems with the bolt hold open! It was holding the bolt open prematurely. In fact, you could lock an empty gun open by simply canting it to the side with the bolt hold open. The problem seemed clear: weak bolt catch spring.

Step 5: get a new bolt catch spring. Alas, not so simple on the DDLES/QC10 lowers. If you install a factory AR-15 bolt catch spring, your empty Glock mags will never lock the gun open. I wound up trimming 3-4 coils off a factory spring, and it was just enough to make the gun work reliably. I may replace it with a QC10 spring in the future.

Does the gun work perfect? No – it still hates Magpul mags, and will not lock them in without really smashing them. That may be a mag catch issue. But I can now run it reliably enough with Glock 33rd mags for USPSA PCC, which was the end goal.

It’s easy to look back on all this and go “why did I bother?” But I also have to remind myself that there was no Sig MPX or CMMG Guard when I bought this lower, and that it’s a pretty reasonable performer now that I’m done with it. If you’re buying a PCC now, there’s no way I’d recommend going with a home build; just get one of the aforementioned factory guns and move on with life.

2 thoughts on “The Saga of How I Fixed My AR-9 SBR”

  1. Colt 9mm SBRs? If any are available when I go to get one a 9mm SBR, should I get one or a Glock mag one. Don’t have a Glock, so I’ll have to buy mags regardless.

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    1. Difficult decision. Glock mags tend not to suffer the crazy fitment issues you see with Colt mags. However, the LRBHO situation with Glock mags is a step removed from being a Rube Goldberg machine and can go wrong fast. The reason I push the CMMG Guard platform using Guard-specific Endomags is 1) smooth radial blowback operation, 2) ejector integrated into the bolt, and 3) no need to screw around with nutty proprietary parts in your lower.

      If you forced me to choose between the Glock and the Colt mags in the end, I’d choose Glock. At least your gun is going to reliably feed and run with those, and missing an LRBHO isn’t the end of the world

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