There was a post recently about barrel vibration and as you can imagine the END of the barrel moves the most. lt sounds like the flash hider being loose is creating some adverse barrel harmonics affecting accuracy.
I run QD flash hiders on all of my rigs for a suppressor and they get installed at 20 ft/lbs without a crush washer using a barrel vise. If using a crush washer the flared side goes against the barrel shoulder.
http://www.itstactical.com/warcom/firearms/diy-ar-15-build-flash-hider-installation/
