I just bought a new Bushmaster 20" A2 upper for my post-ban weapon. I noticed while cleaning it that the rear sight aperture is off a bit, as if it's out of canter. I can move it just a tad and it will spring back into place if I let go. I'm wondering if this is something that will prevent me from properly zeroing my rifle, or should I contact Bushmaster to see if I can send it back in for repairs or replacement.

normal.

My Bushy does it also.  All A2 rear sights are like that.  They are spring loaded and you can kind of "swivel" them around slightly.  They will always go back to where they need to be and it will not adversely affect sighting the weapon in.

Great! Thanks for the info.

If you don't like it you can always order a "high performance" rear sight.  More commonly referred to as "National match" rear sight housings.  Some will weld a small peice of material to the back and then you can dremel off a little at at time until the unit does not twist in the mount. others use springs and ball bearings on both sides and there are other ways as well.

mine A2 rear aperature wiggles back and forth, also.  i just got the rifle a couple weeks ago, and have only been able to test it at about 30yds and it seems fine.  i dont know about longer ranges though.