I've been thinking of building a dedicated 22lr AR and came across this adapter to use M&p 15-22 mags in a 22lr AR. It allows the bolt to lock back after the last round. I already have a bunch of 15-22 mags so it would be great if I could use them in my new rifle.

anybody here use these?

http://www.redi-mag.com/index.php?l=product_detail&p=52

Have not. But getting additonal M&P 15-22 magazines has been a problem all year. I notice the newer 15-22s come with a  10-round magazine that's actually a 25-rounder with an internal limiter and no load assist button. Seems to combine the shortcomings of the 10-rounder (limited capacity) with the 25-rounder (magazine so long that it gets in the way when you're shooting prone or from a bench.)
 

You can use the 15-22 mags in a ciener conversion aka cmmg. It wont hold the bolt open. But you can modify bdm followers to work in the m&p mags.

I've been using this  with my CORE 15-22LR upper/dedicated AR and BHO works 100%. Plus IMHO the SW MP 22 mags are much better than the BDM that the upper came with.  A lot easier to load.  Feeds all the time.

It's a cheap way to practice every week end and with the BMA II, this rifle now has same manual of arms as my regular AR.   Bolt holds open on the last round, eject empty mag, insert loaded mag, close bolt with BAD lever and fire when ready.

SW mag availability is getting better lately.  Got the long 10 rounders from Midway.  Removed the pin makes it 25 rounders and bought some aluminum load assist buttons online.  Couple of weeks ago Midway had the regular 25 round mags in stock.  Also I ordered 4 short 10 rounders from SW, they are back ordered 4-6 weeks. Need them when shooting prone or bench.

I don't use the  BDM anymore.  Also have an extra BMA II.