My barrel appears to be rotating during firing. That is; When I go to zero the BUIS's. I run out of right windage. FSB is clearly rotated clockwise (from shooter's position). This is obvious by casual inspection of the rifle and from noting FSB shadow through ACOG.
Solution: Lock upper in receiver vise and whackerize front sight counter-clockwise with rubber mallet until vertical. Verify by optical inspection and by FSB shadow through ACOG.
Back to the range. Shit! While zeroing BUIS's, run out of right windage again! Observe rifle. FSB is clearly rotated clockwise again.
Solution to solution: Tighten barrel nut one hole. Attempt to re-whackerize front sight. Shit! FSB won't budge. Stuck in obviously clockwise position. Now WTF?
I read what I believe to be the relevant FAQ. It mentioned something about the barrel being installed "cockeyed". I'm afraid I'm not smart enough to know exactly what that means.
Upper reciever: J&T (kinda-sorta FN). Barrel: 20" 1/9 chrome lined, pencil profile, 5.56 chambered Quality Parts (predecessor to bushmaster). A1 FSB and post. KAC 300M BU rear IS.
This gun shoots 0.5 MOA all day long with TA-11 and proper fuel. The TA-11, on LaRue, does not lose zero during shooting, even though the barrel seems (after the fact) to be rotating.
Tweak or somebody help me out here. I'm going nuckin futz trying to get my BUI's squared away.
Thanks in advance.
Assuming that the BUIS is holding rock solid on the upper, and is not loosing it zero,
Pull the barrel and confirm that the FSB is in the same index line as the receiver extension pin.
If the two are correctly indexed together, then reinstall install the barrel with it torque to no less than 35 lbs and no greater than 80 Lbs.
Using a barrel vise block, lock the bore to find a distance focal point. Now look down the sights and confirm that the sights centered zeroed and the bore is aiming at the same point down range.
Now using a Leather raw hide mallet , pull the upper out of the vise, hold only the upper receiver and make the FSB blow at the base to barrel to drift the entire barrel in the upper barrel socket to index the FSB into the correct plain.
Now once the FSB is in the correct index, put the barrel back in the barrel vise block and mark the location of the FSB to the barrel with a pencil. Using a rubber mallet, make a few light strikes to the tower once way, then the other. Confirm that the FSB is holding tight to the barrel, and that you have not penned the taper FSB pins, which are allowing the FSB to shift index on the barrel.
Note: My guess is that the barrel nut was not torque correctly from the start. The bolt locking into and disengaging from the receiver extension was causing the barrel to shift index in the upper barrel socket. The fact that the rubber mallet will not drift the barrel when it is correctly torque is normal. Too much of the impact is lost on the cushioning effect of the rubber.