I posted about this some time back and have been working with this barrel ever since. This is a 16" Bushmaster M4 profile on a Bushmaster flattop upper receiver. I just swapped out a Bushmaster non-M4 profile barrel for the M4. (The original Bushmaster 16" barrel shot great.
The barrel shot about 12" right at 25 yards initially at mechanical zero. With a Bushy carry handle the rear sight had to be cranked almost all the way to the left side to zero.
Well, I thought maybe it might be the FSB/pin alignment thing so I removed the FSB and put on a FSB with a folding front sight-the non-pinned gas block kind-so I could tweak the alignment of the FSB. Same thing.
Next I replaced the removable carry handle and put on a DPMS stand alone rear sight. Same result.
Finally, thinking I just had two bad rear sights, I put an RRA EOTech Dominator base (the one with the built in rear sight) on. I rechecked the alignment of everything with a laser level jig. I leveled the top of the A3 receiver and got everything square. Same problem - 12" right at mechanical zero. Finally I tried the old engineering trick by pulling all the pieces off and trying them one at a time on another 16" barreled upper (J&T) and everything works just fine.
Now I know the pin is probably misaligned. I didn't want to file the slot in the upper - I'd rather have a barrel that shoots true. BTW there is no evidence of the bullet hitting the flash hider. So is my problem simply break in? (about 250 rounds so far) With 6 other AR's sitting in the safe, I've never had one do anything like this. Or, do I need to send the barrel back to Bushmaster?
Thanks in advance for any thoughts on what I might be missing.
It really does sound like a problem fo Bushie to deal with. Clearly you've done more than a reasonable job of trying to fix this thing yourself.