Back in the day, CMMG made their own BHOA which has since been discontinued. I just found out though that they also sold an accessory kit for the mags that had upgraded followers/springs that went along with it.

I'd love to get my hands on these so I can do some testing with them, but they're long since discontinued and it doesn't look like they sold great even when they were on the market.

CMMG ARC Bolt Hold Open Upgrade Kit for 22 Long Rifle Conversions AR-15
https://www.midwayusa.com/product/1003303956


Do any of you have one of these kits by chance? If so, any chance I can buy it off of you?

This is what you need- works great in all my 22AR - dedicated and conversion uppers AR CATCH 22 -22LR bolt catch

Thanks for the link/reference! I'm actually the manufacturer of the Catch22s and that's where this search started. They work great with 15-22 mags for LRBHO, but the thing is that there is still an overwhelming amount of the .22lr AR population that run the 'standard' CMMG mags and don't want to buy a whole other setup. I've been dying to find a way to retrofit the CMMG mags so that they can engage a bolt catch but so far have been coming up short. I'm hoping that getting these old pieces (even if they were a bit of a shaky design), can get me closer to having a better solution for people that don't run 15-22 mags.

That would be great if you get something to work for the BDM magazines. I wish you luck and look forward to your results.

I’ll say it again- excellent and easy solution to holding bolt open.  As the owner of 30 BDM and CMMG mags (mostly BDM) that I fall into the category of not wanting to reinvest for a new LRBHO system. It would be too costly if I had to replace the entire mag.  If you could come up with a new follower that would engage the  Catch22, you’d have many customers (I’m sure this isn’t anything new to you-just sharing my opinion).  
For now, the follower acts as the LRBHO letting me know that the mag is dry (that and the audible difference in the last round that is fired.  One of my dedicated .22 is a spikes M4 upper that I purchased in 2008) the Catch22 is a great work around.  Thanks again for an excellent product.

Thank you! It's so great to hear that it's working well for you.

The way you're using the catch is the way it was actually originally intended. When I made the first prototype, there weren't any 15-22 mags and the core use case was just an ease of showing 'clear' at the range.

I have been working on some prototype followers that generally seem promising. The challenge is though that the stock spring tension isn't strong enough for a consistent lock-up. I'm hoping that if I can get some of the old springs and join them with my new follower idea, it might be closer to a viable solution. We'll see though. I have been working on this problem for a while and there always seems to be new roadblocks!

Except that the follower engagement to the LRBHO is only part of the problem.  At least one other aspect is that the standard JA Ciener, CMMG and BDM mag springs are generally too weak to actuate the LRBHO.  While stronger springs can be installed, in some instances, such stronger springs may cause feed issues as the ammo drag is increased.  Also, over the long term, such stronger springs tend to wear out the mag lips which necessitates using steel lip mags such as those sold by BDM and JA Ciener.

MHO, YMMV, etc. Be well.

How about using a lighter spring in the bolt catch mechanism?  I wonder if that would help or then be too slow to rise and engage the bolt in time.

I don't believe it's a spring issue at all.  The spring on the bolt catch serves to keep the bolt catch held in the disengaged position.  When the magazine follower drives the bolt catch upward, it engages the bolt and holds it open using the tension of the recoil spring on the bolt.  On the 22 conversions, the follower does not engage the bolt catch at all.  Think in terms of the difference in length between a 223/556 round and a 22LR round.

In fact, @alpalwal designed the Catch22 with a small tab to engage the 22LR bolt after it's pulled father rear than the follower of the CMMG or BDM magazine.  IMHO, I think the magazine needs a redesign for the stock bolt catch to work as it was intended to.  I'm not an engineer but my observations of the bolt held open by mag follower shows that there is still lots of travel to the rear of the receiver, thus the ability of the Catch22 to work as a manual bolt hold open device.  

I'm not sure what the solution is but I envision some kind of bar, plastic or otherwise, attached to the follower, that travels vertically within the body of a CMMG or BDM magazine.  This bar would protrude from the distal part of the magazine just enough to engage the bolt catch after the last round.  (sort of like the tabs on a 9mm colt SMG mag that are widely used in Colt pattern 9mm PCC).  Anyway, just my thoughts on this since this thread has gotten me thinking.  I love shooting my 22ARs and would enjoy any improvement to the system.

You mean something like this:
https://pdfpiw.uspto.gov/.piw?PageNum=0&docid=04069608&IDKey=645D3B5779B5%0D%0A&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D4069608.PN.%2526OS%3DPN%2F4069608%2526RS%3DPN%2F4069608
IIRC, that's the Air Force .22 conversion system for the AR platform - not full auto capable as is, but can be made so.

Also, take a look at an Olympic Arms PCC mag - very similar.

MHO, YMMV, etc.  Be well.
ETA: I have an Olympic Arms .45 ACP upper and mags. The mags have very stiff springs and the LRBHO is very positive.

Ya, IDK, my only experience is with the Boonie Packer and 15-22 mags so it is Greek to me.  Just wondering out loud.

That USAF magazine is pretty cool.  Was that a flash in the pan that didn’t catch on or was it one of those $500 toilet bowl seats from the 80s.

The Air Force used the .22LR conversion kits for rifle training instead of 5.56 ammunition. You can still find magazines on ebay.

I may have what you are looking for, I bought a CMMG upper several years ago that came with the bolt hold open device and 3 mags that the guy said we're upgraded with the springs and followers. I believe the followers are black, does that sound right?