I have both a Henry Golden Boy and an M&P 15-22. Tough I have no data to back this up in the way of measured DB levels, my experience with these rifles is that (unsuppressed) the 15-22 is marginally louder in operation.
I can fire both outdoors without hearing protection to no ill effect. (Full disclosure: I used to play in a band, so I can't hear worth a damn in the best of conditions anymore.)
Oh, if we're showing off anyways:
I purchased mine to serve as a serviceable training tool for the AR15 platform. I sold the .223...but kept the .22
Thompson Machine Zephyr XL....high quality, low cost, great suppression. Price is going to go up substantially if you're looking for .223 suppressor compared to a .22 suppressor, but you'd be able to put it on multiple platforms.
I love mine however three things to consider for as my research has found
1. Make sure it has a blue spring on the bolt. This mean it has been updated to latest specs by S&W
2. Mine will only reliability fire and feed CCI ammo no bargin box ammo for this rifle.
3. Make sure when you load the magazine you stagger the bullets as this causes issues.
If you don't mind those three points it is the closet to a .22 AR in function and form you can get. Trust me I looked around for 6 months before I bought mine.
Yes the blue spring fixes the issue. Mine outshoots my brother 10/22 all day long. Not to mention AR parts fits this gun. It's the gun to get. The only regret I have is not getting the MOE edition. If you like magpul stuff it's worth the extra 100 dollars.