I found this on the Bushmaster web site a few minutes ago:
This is a problem that took us some time to figure out but it is so simple, we're kicking ourselves. The little index pin that lines up the upper and lower is very soft, so that it can shear off if the barrel is over-torque. This is to prevent damage to the receiver when barreling. This pin will distort slightly before it snaps, and this slight distortion is enough to create the excessive windage you describe. A new pin costs only $1.50 and removing the old pin is as easy as grabbing onto it with vise grips and twisting it up and out. Drive a new one in and try the upper on. The pin may shear when you try to remove it, but they are easy to drill out. Just be sure not to drill into the barrel extension. A new pin will be like having a brand new barrel (as far as the receiver is concerned, it is new) and that windage problem should disappear.
Thought this might help you...might be your problem although I have heard several folks complaining about this exact problem with Bushmaster barrels. I have somewhat the same problem only my pin is not bent or distorted. And I'm sure I didn't over torque the barrel nut.