Winchester just released a new subsonic .22LR round announced at SHOT this year. Purportedly is 1,060 FPS for 16"-20" rifles, with a significant noise reduction compared to normal rounds. ("Standard" velocity .22LR at 148 dB, M-22 rounds at 129dB.) Note that this a 45 grain black copper plated round nose bullet that has been "optimized to function in semiautomatic firearms" and is "ideal" for use in suppressed weapons. Since this is so new, there's not much real world info out there that I could find, and thought you guys might be interested.

Many of us, I suspect, shoot the M&P 15-22 as a dedicated AR trainer. It's a bit picky about its ammunition, so I almost exclusively use CCI AR Tactical 40 grain copper plated round nose rounds, rated at 1200 FPS and optimized for this weapon. These are often hard to find. With a box of Winchester M-22, I set out today to assess:
(1) Ability to cycle reliably in the M&P 15-22 as a subsonic round
(2) Noise
(3) Accuracy

Rounds were fired through a non-suppressed M&P 15-22.

Only had a little time, but after firing a little over 100 rounds, the somewhat crude test results are:
(1) When the rounds fired, they cycled every time. However, I had a 5%+ dud rate- about 1/magazine. Seems a little high in comparison to other brands.
(2) Noise was indeed a little less than normal velocity rounds, but no where near a suppressed rifle.
(3) Accuracy was roughly comparable to AR Tactical in 5 shot grouping at 25 yards with a basic red dot in the prone position. (Admittedly not ideal for the real target shooters out there. I may test in my scoped bolt gun later.)

For me, I was hoping that I could substitute the M22 as a quieter round that I can actually find and buy locally for a little over 7 cents/round. Though it is a little quieter, I think I'll still keep on trying to find CCI AR Tactical when I can as my go-to round, or perhaps alternatively, Federal Target Grade Performance bulk rounds (40 grain/1200 FPS).

Hope this info can be of use!