The problem is that your barrel assembly is slightly misaligned with your upper receiver.  In order to correct the problem, you'd need to remove your gas tube, then loosen the barrel nut, reindex the barrel, then retighten the barrel nut.

I can do this myself in about 5-10 minutes, BUT that's because I already have the tools.  You'd need:

- a vice
- action blocks
- punch for gas tube roll pin
- barrel wrench
- torque wrench

Any AR 'smith should be able to fix this for you for fairly cheap (much less than buying the tools).

-Troy

Bring your upper to Knob Creek with you.  Duh.  

-Troy