Another vote for the Smith M&P 15-22. I’ve had mine for several years and have no real issues with it. The occasional stoppage (usually failures to feed) but not enough to cause me any heartburn. Like you are planning I set mine up similarly to my “work” guns so the training crosses over.
Another vote for the S&W M&P 15-22 .
Some advise for someone looking to buy-some of the earlier ones had problems .
The main spring and the extractor were changed a few years ago .
Either look at the box for the production date and verify that it’s 2014 + , or
look inside the rifle and look at that main spring behind the bolt .
If the spring is BLUE it’s got the new spring and extractor .
Last I heard they were making the spring blue in every new gun
built after the revision .
I had to send mine in and they replaced the entire bolt and spring .
Eats bulk ammo like nobody’s business .
If I could still get Federal 550 bulk packs I would already
have a second 15-22 and some .22 dueling trees to shoot
when the guys come over .
Another Happy M&P 15-22 owner.
I tried many other .22 trainers including the drop in conversion kits and even built a complete .22 AR using a Tac-sol dedicated complete M4 upper. It was fussy with the ammo I fed it. I have a 10/22 but it is not the same as an AR. I tried the SIG 522 and it was a fine rifle but the controls are not close enough to an AR and then there was some odd issues with C clips and how the recoil spring attached to the bolt. Old news but I sold it anyway and got an M&P and never looked back.
The M&P is the way to go because:
Reliable with everything I feed it including ancient mystery box of loose .22 from my father.
Mimics the controls of a 5.56mm AR very well.
Accurate enough for training and fun as hell to shoot.
Plenty of aftermarket parts and support to turn it into almost any type/version AR you want or make it as close to one of your 5.56 rifles.
Magazines work and are easy to load.
Inexpensive magazines
Makes a great suppressor host.
Inexpensive cost of entry into the platform
Well made and lightweight so kids like them.
Polymer construction is excellent without feeling cheap like some of the pot metal .22 rifles.
Accepts standard AR fire control parts.
I don’t hesitate to tell my friends or anyone else who asks me about the M&P 15-22 to buy one.
The question is why don’t you have one already. 
I have a Sig 522, which is not a good trainer, but it is a very reliable .22lr. The Sig 522 battery of arms is different than an AR so it is not great for serious training. But as a reliable .22 it is terrific. BTW - Because of the lead fouling issue I try very hard to use copper plated .22 rounds, and I prefer CCI.
The reason I’ve not bought an AR pattern .22 is because I keep hearing about problems with reliability. Maybe that has changed in the last 3-4 years. Maybe I’ll check out the M&P. My 1 cent worth.
I disagree with the entire premise of the training value of any .22. YMMV but I would spend that money on reloading equipment or ammo.
If you need general marksmanship training go get yourself a 10/22 or bolt .22 and learn how to shoot. Trigger time is trigger time, regardless of the platform. However, that doesn’t sound like what you are looking for, which is why I make the statement above.
Of course, not all will agree with this opinion.
I dearly love our M&P15-22!
I bought it and set it up just like my Colt 6920. Magpul furniture and a Primary Arms Aimpoint knock off. Bought it with the intention of having a 22 trainer that was nearly identical to the 5.56 Colt set up I already had. Bought it maybe 3-4 years ago.
Since then…we have shot the daylights out of the thing. It is accurate and very reliable. Easy to clean and as long as you keep the mags pretty clean it almost never jams.
Up until 22LR up and evaporated around here, we used to run bricks and bricks through this rifle. My daughter and her friends used to take it on the ATV and go out in the woods and blaze away at cans and targets. They get it really dirty and it got left in the rain a few times. Still…the M&P15-22 is still ticking!
Yes…for a 22LR trainer, it is hard to beat the little S&W M&P15-22.
Big…big fun.
(…if you have a goodly supply of 22LR…which is hard to do these days)
It has value, but also generates some bad habits if you train exclusively with it. I don’t think the damage is as severe as in a pistol, but even if the controls are the same, actual weapon manipulation is nowhere close. I rarely use mine since getting a progressive reloading press.
15-22’s are awsome. I’ve got 3k or so through mine and I’ve never cleaned it…it is absolutely disgusting. I hose it with Slip2000 and feed it whatever bulk .22 I can find. Get one.
I, too, have the Tacsol dedicated upper (AR22) on a standard AR lower (old BM) but mine functions fine with a Geissele SSA.
I’m suprised how expensive the dedicated uppers are compared to the M&P. I thought it would be the other way around.
I went a different route. I researched the crap out of it and decided on the 15-22. At the last minute I switched gears and went 9 mm ar. I set it up the same as my primary ar, including stock, handguard. Weight and balance are very close. Recoil is a little lighter but closer than 22. Reloading is the key to make it a viable option.
I have thought about doing this but found the recoil to feel slightly heavier with the blowback 9mm.
For as much as a shoot my tacsol .22 and as much as I have wrapped up in it, I probably would have been better off just to get the cheaper S&W.
Another vote here for the M&P 15-22; remarkably reliable and so fun to shoot and as mentioned previously can be configured to mimic your 5.56. I use ours at our clubs steel match on occasion and its a blast. The mags are very sturdy as well and certainly allow for good malfunction and reloading drills. You do have to modify a BAD lever or the lower at the trigger guard if you want to employ one of those devices though.
I built a dedicated .22 upper for my SBR because I couldn’t find a commercial one that was exactly what I wanted. And after having it a while I ended up getting another lower and SBRing it so I could just make that a dedicated .22 gun. I didn’t like the idea of my defensive gun getting all that . 22 crud in it all the time. The moral of the story is: if you want a .22 AR get a dedicated gun, and the M&P 15-22 is definitely the best balance of cost and quality.
The .22 AR is great for new shooters and for some cross-training, but without the full recoil of a 556 I think it’s training qualities are limited. However, it sure is fun to shoot! I use mine a lot and it’s especially fun with the suppressor on it.
Will the threaded barreled m&P 15-22 mate with the Spectre ii silencer?
Yes.
Both come standard with 1/2 x 28 threads.
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“A few years ago I ruined a 5.56 barrel using a .22 conversion bolt. Put a “ring” in the barrel (from a squib .22 then firing another .22 behind it). Before you jump all over me “should have assesed the failure” blah blah blah you should know that a squib round on a .22lr will happen sooner or later just due to the nature of Rimfire, when it does happen you do not hear anything it sounds just like a dud (which are common on Rimfire, so you get accustomed to racking them out and moving on). I was “on the clock” on a stage, I racked it out and keep competing that’s how it happened; didn’t know anything was wrong until I cleaned the gun later. If you are around .22lr rifles long enough you might see some that have “ringed” barrels and still seem to shoot fine (sometimes owners are not aware that it happened) and usually don’t hurt anything, but given this was a 5.56 I couldn’t take the chance with 5.56 being fired later.”
Me too. Had a CMMG unit: mediocre accuracy, iffy reliability, and it finally cost me a barrel from a bulge. Live and learn. Bought a 15-22 about three years ago and love it. Coming up on 4K rounds through it, with zero issues other than the usual occasional .22LR misfires. Put a spare Jard trigger in it (the stock trigger actually wasn’t actually too bad). Put a Bushnell TRS-25 red dot on top. Sure, it’s lighter than a real AR, but I’ll tell you what–it’s a whole lot like my 5.56 that mounts a compact ACOG.
I wish everything I bought was half as good.
Back to the OP question, I would say get just a 22lr upper with a standard mag 22lr that works in a .223 lower. Personally I have a drop in and it hasn’t failed me yet after 30 years. Just run copper jacketed 22lr through a drop in. And then shoot 1 round of 5.56 to make sure the gas tube isn’t clogged (never has been for me). Drop ins and interchangable uppers are great and affordable and don’t comprise of plastic like a M&P 22. Or just build a new upper and leave a drop in on it and never use 223 in the gun so you dont need to care if its gas port gets clogged by the lead.
I bought a CMMG dedicated upper and I love it . I don’t even train for anything it is fun and everyone that tries it hands it back to me with a smile on their faces. The dedicated upper keeps the exact AR feel as the full caliber ARs.
I fooled around with a few dedicated uppers for my sbr. The tacsol is the cream of the crop.
That said, I eventually SBR’d a 15/22. With a 7" barrel & an SWR Warlock2 on it, it’s about the same length though lighter than my 12.5" barreld BCM Kino AR. Somewhere past 5k rounds through the 15/22 since new. It runs like a sewing machine!