You'd have to remove the bottom lug that runs the length of the M&P15-22 lower receiver extension and glue the M16 stock onto the tube straight as there's nothing to index it. Get it on cockeyed and it's cockeyed for life.
I'm not sure what's under that lug and the lower receiver is permanently cobbled by this modification.
Note the M&P15-22 lower receiver is longer than the M16 so the M16 stock will be longer on the .22 rifle than the real one. About the same length as the telestock all the way out which is a good inch too long for me.