I just read in another thread that if the ar is not accurate then try retorquing the barrel.  Is that true?  If yes, then how much torque do you put on the barrel nut?

Quoted: How much torque do you put on the barrel nut?

To answer the last part of your question, torque the barrel nut to 30#, then tighten as much more as needed to clear the gas tube through a notch in the barrel nut.

Retorquing the barrel nut will only help accuracy if the barrel is loose.  Are the shots all over the target, at random, perhaps even missing the paper?  If so, it could be a loose barrel.  Is the group just larger than you want?  If you are shooting 2"-3" at 100 yards this may not be a tight "braggin'" group, but you would not get that with a loose barrel.

If there is at least 20 ft-lb on the barrel nut, the barrel will not be loose.  The minimum should be 31 ft-lb.  Once you reach 31, take off the torque wrench.  You don't need it after you reach that minimum.  Now use a long breaker bar (more leverage, finer control over slot position).  From here, you simply keep tightening the barrel nut until you can pass with gas tube through with equal wiggle to both sides.  What the torque is to achieve this is unpredictable.  There is no magic torque figure where it will line up correctly.  Just get it lined up by tightening the nut, do not back off to achieve alignment.  What the final torque is, it is.  You don't need to know, you don't want to know.

SHOOT FOR 80  : )  

No honestly though, my issue Colt M4 loses zero by inches at 25meters regularly, and my barreled upper that I brought along hasn't lost a zero since it was built (that is 3 years with me and 3 and a half months of combat abuse.  A frikken 50cal ammo case [two cans in the wood] nailed it at one point for petes sake.  IT IS TOUGH.  I am a firm believer in over-torqueing them.

One of the guys in my platoon's M4 has a barrel nut that has loosened enough to make mounting his 203 very daunting.   It is visibly off of centered and wasn't before.  Colt QC SUCKS