you have been asking some very ridiculous questions which leads me to believe you have almost zero mechanical knowledge. my suggestion to you is that you STOP and read some of the stickies or get yourself an instruction manual. asking these types of questions indicates to me that you are not doing things properly. i'm not trying to insult you, but it seems you are blindly trying to assemble a rifle or rifles and if you aren't doing things properly, you could hurt yourself or damage your rifle.
just like your question about how much torque it would take to "stretch the threads on the barrel by tightening the muzzle device"...
my response here is you are once again improperly assembling something if you are worried about "how much torque is too much". the castle nut should be torqued to 40+/- 2 ft lbs per spec sheets...no more, no less. why do you care how much torque beyond that will break something? you shouldn't tighten beyond torque specs, torque specs are there for a reason.