Your gunsmith did not properly center the front sight and torque the barrel. Barrels should be installed per the instructions in TM 9-2320-319-23&P. This manual shows two different methods of centering the front sight, (the Army uses one method and the Air Force another, go figure.....) then it is important to torque the barrel nut 3 times. Turn it tight (to the in pounds specified) then back it off and repeat twice. Also, never loosen the barrel to line up a notch for the gas tube, just tighten.