Take off the muzzle device and shoot it. If you still have problems check the crown. If it shoots without the muzzle device it was on too tight. If using shims find the right combination to just get it snug and timed correctly. If using a crush washer you'll need a new one, but same thing, just snug and timed correctly. I work for an AR 15 pattern rifle manufacturer, and it's usually these two things. You can get a bad barrel though, usually flared slightly over bore/land diameter at the muzzle. Anything over 0.005 inches will start affecting group size.
