Great guestions,
Im in pieces now waiting for a handguard.also recently added a wilson combat upper side charger and i had to rat tail a little material out of the pin holes to mate the S&W lower. With these questions leads me to believe the upper is 3 or 4 thousands to far forward from the buffer tube. BTW the tube in screwed all the way forward. I will come back with some pic’s when shes back together. Thanks for the help, though ill need you guys a little more.🤠