I  took  the  barrel  off  a  s & w  m-p  15-  22  how  many  foot  pounds  do  I  tighten  the  barrel  nut  up  2  ?

I would reach out to these guys. They modify these all the time: Tacticool 22

If you read their paperwork (tacticalcool22) that they send with their barrel nut removal tool, it states to call SW for the torque specs. Also, they don't provide the information any where on their website either.  SW will not provide the information if you trying contacting them.  In fact if you go to the smith Wesson forums there is a constant stream of people asking this very question without anyone who can definitively answer it.

Having said all that, just torque it tight.  I have taken my barrel off a few times and never had an issue with it shooting incorrectly after tightening it firmly.  Remember that it is plastic and metal and you need a barrel clamp to tighten it.