There's no pretravel or take up on the M16 or AR15 trigger. It's a single stage trigger. The sear is moving as soon as you apply pressure to the trigger. Actually the sear is the front edge of the trigger interacting with the hammer notch.
I'm pretty sure the M16 was the first (if not only) US military rifle to have a single stage trigger. Even my muskets have take-up before the trigger starts to move the separate sear.
A two stage trigger had movement before the sear starts to move.
Anyway, the trigger in the M&P15-22 is stock AR15 and single stage.
-- Chuck