This has to be a two part post as I'm a relative newbie and am limited in the size message I can post. This is part 1. I hope I'm permitted to follow it immediately with the second part. We'll see.

I recently decided to build a dedicated .22LR AR upper. From CMMG, I purchased a .22 bolt and barrel group and a couple of 22ARC .22 conversion magazines. These look much like ordinary AR mags but have internals and a top designed to work with their .22 LR bolt and barrel in a standard AR upper..

Since the upper I'm building has a shorty 4.5" barrel (I'm planning to hide a suppressor under the handguard), I have to use it with my one and only SBR'd lower, a Spike's Hellbreaker.

I'll tell you what I told CMMG:
____
I recently purchased two magazines, 22ARC, 25 rd, on order xxxxx.

Curiously, the magazines will just fall out of the magazine well on my Spike's Hellbreaker lower receiver and on a New Frontier Armory polymer lower receiver. They are, however, properly retained in two much older AR lowers (Olympic Arms). I normally use a combination of PMAGs and Hex Mags in the Spike's and NFA lowers and I have never had a problem like this. Your 22ARC magazines seem to be a bit too skinny to be retained in the Spike's and NFA lowers.

Part 2
As I was saying to CMMG ...

Your 22ARC magazines seem to be a bit too skinny to be retained in the Spike's and NFA lowers.

I had to add a shim (about .035" thick) to the right side of each 22ARC to get the magazine catch to engage with the Spike's and NFA lowers and be retained. And, despite the shims, they do drop free from these lowers. Interestingly, even with the shim affixed to the 22ARC, they still fit into, and drop free of, the older Olympic Arms AR mag wells, further suggesting that they are too thin to begin with.

My concern now is that the shim has offset the magazine a tiny bit to one side and I may have feeding problems.

I suspect that, possibly, the Spikes magwell may be a bit wider internally than some others and the mags seem to be a bit skinnier than ordinary AR mags. This combination fails to retain the magazines. (The mags are not as loose in the NFA lower as in the Spike's, but still not tight enough to be retained.)

I have only four AR lowers to test but you may wish to run some tests of your own against assorted different lower receivers to see if the mag catch is able to retain them. My newer lowers cannot, but two that are decades old will hold the 22ARC. You might want to look into it.
____

I've been looking to try and find, without success, actual Milspec dimensions so I can confirm my "skinny" hypothesis.

Comments?

Are these the Gray CMMG 22 long rifle mags which come with the CMMG conversion kit?   The latest CMMG 22 conversion mags I purchased had a rib on the rear with nubs.   The nubs were molded into the spine of the mag.  You needed to file the nub so the mag fit your rifle's mag well.   If you think your CMMG mags are defective call CMMG.   I think you'll get a quicker response from Customer Service over the phone than by email.  I wouldn't be surprised if the magazine parts are provided by a vendor to CMMG.  I've encountered QC issues with the molding of the mag bodies.  This was easily corrected with a utility knife.  I sent CMMG an email giving them a heads up and never got a reply.


I've got five of the Gray 25 round CMMG  22 mags with green followers.   Once I worked on them they have been GTG.  I have many more Black Dog Machine mags which are GTG.  Many people with CMMG bolts are buying the Boonie Packer Better mag adapter and using S&W 15-22 mags.  Using the Better Mag Adapter with S&W mags gives a fully functional bolt catch like a centerfire AR-15.  The BDM and CMMG have bolt hold open followers.  When the last spent case is ejected the follower rises and keeps the bolt from closing.  Withdraw the magazine and the bolt closes, many users prefer the fully functional bolt catch provided by the BMA.

Better mag adapter and go get a bunch of m&p mags, they can be found for Cheap, are way way easier to load, has the last round hold open. Second favorite is blackdog steel lipped gen2? mags, then the cmmg grey mags, but you have to trim the flashing off the insides, and look for any catch points in the follower track in my experience, and do a little filing on the side tabs. If they are the black cmmg evolution skeletonized mags, just toss then and don't look back, I never got those to work right. I love the cmmg kits though, bought three.

i would say that the m&p mags are looser in my lowers than the cmmg though, but it hasn't hampered reliability with any of my kits or dedicated 22lr ar. And seriously, the loading ease cannot be overlooked as their best selling point.

A Rock River 9mm mag catch has more reach and is inexpensive - $6.

Rock River 9mm mag catch

No. Black polymer with orange followers.

The problem you describe is the opposite of what I had. Yours were too fat front-to-back, mine were too skinny side-to-side.

Thanks, I'll look into that.

Sounds good.

Sounds less good.

Okay, I bought one of the Better Mag Adapters and some S&W 15-22 mags. Unfortunately, it doesn't work.

First, the adapter would not fit my Spike's Hellbreaker lower. I had to do some hand fitting to get the adapter into the magwell. (The BMA instructions suck.)

Once the adapter was installed, it would not function, by which I mean that the adapter's bolt catch was too tight in its slot and would not move. One moving part and it didn't work. I can only assume that the tightness of the BMA in the magwell is squeezing it closed enough to make the bolt catch bind. More hand fitting.

Finally, after getting the bolt catch freed up, I found that the upper cannot be properly mated to the lower when the BMA is installed as directed. With the pivot pin installed and trying to close the weapon, the upper hits the BMA and there's about a 3/8" gap between upper and lower just above the take-down pin. Yes, it can be closed, but it is against a substantial resistance of the BMA interfering with the upper. Removing the bolt is no help, so the interference is primarily with the upper, not the bolt.

If I lower the BMA in the magwell enough to eliminate the interference with the upper, then the magazines cannot be inserted far enough to be retained by the mag catch. Obviously the insertion position is critical and, being so, it's surprising that there is no hard stop at just the right position.

The BMA seems like a cool gadget, but it does not get along with my Spike's lower.

I'm curious to know, with the BMA installed, should I be able to use the bolt hold open manually as before? Or does it only supply last shot hold open? I ask because I cannot get the bolt hold to work when manipulating it manually.

Make life easy on yourself. This inexpensive part give more reach.

So you've said. Twice.

Since the link you provide has no photo of the item you recommend, I've no idea what it even does.

And since this is a .22LR conversion, not a 9mm, I'm left wondering why you think I need one of these mysterious items.

Don't be snippy newbie. I'm trying help you. 

Its a mag catch, looks like all others. Just a little bit of extra reach. Part number 9MM0099. Read the link I sent you.

Since your mags are falling out, there likely isn't a good hold on them from the catch. This extra reach will give you more catch material in the slot on the mag. 

Try this. If this doesn't work you aren't going to find much other that will help. Seems your lower is out of spec.

Then BE helpful. "Get one of these" without saying how it will help is no more helpful than telling me to swing a dead cat over my head ten times while standing on one foot.

  This is the entirety of the info on this part at the link:  "9MM0099 9mm Magazine Catch $6.00"  

Now, that's helpful. Unfortunately, being a lefty, I'm looking for am ambidextrous mage catch. But thanks for the advice.

So the bolt catch in the upper goes into (not on top of, or over) the better mag adapter, mine is obviously filthy, but it should look like this.  How I do it, Remove the steel bolt catch adapter thingie from the BMa , insert the bma frame from the bottom most of the way, fit the adapter thingie over the bolt catch and then slide the bma up the rest of the way. You should be able to use the bolt catch as normal when the upper is closed. You can also just leave out the bolt catch adapter, the bma just sort of floats over the mag catch, so I have to press the mag release when sliding the frame up the magwell, that works easiest for me, YMMV.

Attached File
Attached File

I'm happy to say that I found the interference between the BMA and the upper that was squeezing the BMA and causing it to seize up. A tad more hand fitting and the BMA now works as advertised on my Spike's lower.

Thanks to those who recommended the BMA. Cool gadget.

I use the better mag adapter and run mp 22 mags

Feeds perfect, locks bolt back, no issues