Thinking about building a dedicated 22 ar15a2 style rifle(using Cmmg internals)but before I start collecting parts had a few questions

Sights:
Does anyone make a peep sight insert that is similar to the tech sight peep sight? Specifically, they use a non same plane sight that has a small aperture zeroed at at 25 and then you flip it for an ~75 yard zero.

If not, anyone have a quick math setup for determining zero clicks for a 22 when using the standard a2 peep?(i.e. 5.56 aight at 3, 4, and 500 meters would correspond to which 22lr distances?)

Magazines:
Would like a fully functionally bolt hold open with “20 round sized” magazines; is best bet the resi-mag conversions with 15 rd 15-22 magazines or is there any alternative for aluminum .22lr mags that are sized the same as 5.56 20 rounders(primarily for prone shooting)

Sorry I can't answer your question about peep sights or clicks...I have one 20" AR22 w/CMMG Bolt/Barrel that I have Lyman target sights mounted with Lipski bases.  It's pretty slick.  Otherwise an A2 carry handle sight might suit your needs.

Others will disagree I'm sure, but I've never had any luck with CMMG branded magazines.  Black Dog magazines are the standard but the bolt is stopped on the follower then closes when you drop the mag.  Not what you're looking for.  You're on the right track with the redi-mag adapter and S&W MP 15-22 magazines.  S&W offers a 25rnd and 10rnd magazine.  Midway has them in stock now.  I have many of these and they work great and the BHO function is near 99%.  An alternative to the Redi-mag adapter is the Catch22 device.  There're two types; the first replaces the bolt catch on your lower and the second attaches to the std mil-spec bolt catch.  I have one of the former and it works great too.  I do not have one of the latter.  With the catch22 device, you'll need to shim the S&W magazines.  You can see that they offer two types of shims too.  Stick on the magazine or a clip that fits on the front of the magwell.  I use the clip so that I can still use my redi-mag adapter and un-modified mags on other lowers.

Kriss offers a bolt hold-open nearly identical to the catch22 and works with the CMMG bolt too.  They also have a polymer magazine that has a bolt hold-open follower that is full size.  No shims or adapters needed.  These work great too though the spring is a little lighter in the Kriss mags and the kriss bolt catch comes with a lighter spring.  Similar to the original Catch22, these basically make your lower a dedicated AR22 lower.  If you don't want to do that then the redi-mag adapater or the removable catch22 V2 is what you need.

Black Dog also makes a 32rnd magazine that has steel feed lips and also offers a bolt-hold-open follower though I have no personal experience with that one yet either.

I believe compass lake builds a A2 upper 22. They have a long history supporting the A2 center fire national match rifles, and just getting one of these may be the “ easy button” I think they also offer aperture rear sights with replaceable inserts.

I've also wanted to do something similar. In for the updates

I looked at the cle kit and it says it’s based o the m261 kit. My search has not been productive if this is the same as a ciener kit and can be converted for bolt  hold open

@Erik72 I appreciate the in-depth response, you’ve given me a lot of info to research

The M-261 conversion kit was used by Army National Guard units to shoot M-16-A1 rifles at indoor or outdoor pistol ranges.   The M-261 is a completely different design that uses a ten round clip inserted into a STANG 5.56 magazine.  Black Dog Machine manufactures a 25 or ten round magazine compatible with the M-261 22 LR conversion.  Olympic Arms manufactured a copy of the M-261 and marketed it as the RFA (Rim Fire Adapter).   The Olympic RFA used the same ten round Clip or BDM magazines.

Keystone Accuracy reportedly sell accurate 22 LR barrels and are designed to use a modified Atchisson/Ciener/CMMG pattern bolt.


Here is a link to Keystone Accuracy

https://keystoneaccuracy.com/ar22/

Here is a picture of a M-261 22 LR conversion kit.   Notice the three metal ten round clips and the BDM polymer magazine at the top of the photo.

Other than Compass Lake Engineering (CLE) and Keystone Accuracy I only know of three other possible sources for a 22 LR 20” barrel or upper.   Right to Bear sell CMMG compatible 22 LR barrels and uppers.  RTB list a 20” 22 LR barrel but it has been out of stock over a year.  CMMG no longer list a 20” 22 LR barrel.   Rock River Arms sell a stainless steel 20” National Match training upper in 22 LR.  There are a couple of reviews on rimfire central but the forum members had to order the uppers and they had long wait times before delivery.  The RRA uses a stainless steel barrel and a Nordic pattern bolt.   The only other possible source is Model 1 Sales.  Model 1 sales list 20” 22 LR barrels and uppers but I’d call to see if their barrels are still available.  The Model 1 Sales use a .500” collar whereas CMMG/RTB use a .460” collar.  Model 1 Sales bolts looked to be Ciener but again I’m not sure if they still have 22 LR bolts for sale.   You could use a CMMG with a .500” collar but Model 1 Sales got very mixed reviews on the accuracy and quality of their 22 LR barrels and uppers.

Here is a list of all the current 22 LR suppliers of 22 LR barrels or uppers except for CLE and Keystone Accuracy listed above.   If you want a 20” barrel your only other option is to have a gunsmith turn a blank into a 22 LR barrel.

"Here is a link to Rock River Arms national match 22 LR upper.


https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=1993


JP Enterprise 18” 22 LR barrel and bolt

https://www.jprifles.com/buy.php?item=JPSM22-18L16

Beyer Barrels 18” barrel is longest length available.

http://beyerbarrels.com/barrels/ar15_barrels

Nordic Components

https://nordiccomp.com/categories/nc-22rb-upper/

Model 1 sales 20” upper

https://www.model1sales.com/index.cfm?fuseaction=category.display&category_id=283

Model 1 Sales 20” 22 LR barrel

https://www.model1sales.com/index.cfm?fuseaction=category.display&category_id=366

Right to Bear 20” 22 LR barrel out of stock

https://www.righttobear.com/rtb-20-inch-22lr-ar-barrel-rtb22lr20gp/

TACCOM3g 22 LR

https://taccom3g.com/product-category/for-ar-15-22lr/

Palmetto State Armory out of stock 22 LR

https://palmettostatearmory.com/ar-15/barreled-upper-assemblies.html?caliber_multi=.22LR

CMMG 22 LR no longer list 20” 22 LR barrel

https://cmmg.com/barrels

Have you decided which direction you plan to go?  ....just curious.