I came into an extra lower a while back and decided it would make a good platform for a dedicated .22LR. I put together an upper with some parts bin stuff and a CMMG barrel and bolt kit. This thing is stupid fun, and crazy reliable. The only problem is that the conversion bolt can't engage a mil-spec bolt-catch, and as such there's no way to lock the bolt open.

I use the Black Dog Machine mags that hold the bolt open on the last round, but when you remove them the bolt goes home, and there's no provision to lock it back. I have some safety concerns with not being able to see if the gun is clear, especially because I've been using it to teach my kids. Our range also requires chamber flags for any unencased firearm that's not actually in use, and that's a pain when the bolt won't stay open.

The other day I was trying to think up some kind of home-grown solution, did a little googling, discovered that somebody already figured it out: https://www.righttobear.com/ca...ch-for-22lr-catch22/. It adds an extension and a tab to the front of the bolt catch so that it will engage the conversion bolt. I still have to manually engage it when I remove the empty mag (there's a way to even accomplish that with an adapter and M&P 22 Mags), but at least now there's a way to keep the bolt open.

Haven't had a chance to shoot it yet because it just showed up this evening, but dry it works as advertised. Just thought I'd share in case anybody else has a similar need for a dedicated .22 build.

Don't know how invested you are with Black Dog mags but I really like the S&W mags.

Instead of the Catch 22 bolt catch I used the Better Mag Adapter https://www.redi-mag.com/produ...re-magazine-adaptor/ along with the Catch22 S&W 15-22 Magazine Adapter for Standard AR Lower - (Clip on) https://www.righttobear.com/ca...lip-on-catch22-clip/.

The nice part about this setup is that the 2 added parts are removable from the lower with no tools and your lower can then be used with a 223 upper and magazines.

Have mine setup with a dedicated CMMG upper and it looks and feels like my Colt 6920. Its a blast to shoot especially steel plates.

Also have thought about a Franklin Binary trigger for it for a little extra fun.

I use the same thing. Mine is the Boonie Packer mag adapter so I think it is the same thing. They work great and 15-22 mags are way better than Black Dog. I have both and it’s no contest.

Make that three. My 8.5” SBR’d dedicated .22 runs on the S&W mags exclusively now.

I may eventually give the M&P mag adapter a try. Unfortunately I'm already invested in the BDM mags, and they work, so it's not high on the list of priorities especially now that I have some means of keeping the bolt open. This thing is stupid fun to shoot, and I can't believe how reliable it is. It would be REALLY fun in full auto, because you could actually control it.