By the way, you might be able to avoid this whole SBR hassle altogether. Tactical Solutions makes a
shrouded barrel for the 10/22 called the SBX; it has a normal barrel about 11 inches long, then it has a shroud the brings the overall length up to 16". You can slip a .22 suppressor inside and screw it on, and the suppressor only adds a tiny bit of overall length. It works best with a Sparrow; the Sparrow is wider and shorter than most .22 cans so it fits perfectly snug and only sticks out a 1/4 inch or so. The Spectre also fits great, but it's not as snug and it sticks out about an inch or so.
You might want to see if anyone makes a shrouded barrel like this for the M&P 15 .22 before you go to all the trouble to SBR it.