Has anyone run into this, and what was the problem?  Sights are snug, not loose, and front sight housing is spotwelded to the barrel.  Its a Frankengun 16" A2 carbine, supposedly mostly Bushmanster and DPMS parts.  Any help would be appreciated.  Ops

As my dad would say, "must a be a loose nut, between the seat and the steering wheel"...[;)]

Seriously though, if the rear A2 and the Front sight tower are NOT moving then your head/eye, behind the sight must be. Make certain your getting a consistent cheek weld on the stock and unless ya have an eye dominance problem, (such as your left is dominate, but your shooting right handed, in this case close the dominate eye, as it'll mess up your sighting) shoot with both eyes open.

Mike

Could the barrel crown possibly have gotten damaged?

what's spotwelded to what?

Same ammo from group to group, or different day-to-day shooting, using different lots of ammo?

Oops, this mainly appears to be a shooter problem, different eyeglasses DO make a diff in sight picture.  Astigmatism changes as the lens is rotated, and bends the light differently.  I plain english, I cannot use mu scrip sunglasses for shooting until I have them corrected for the angle.  And yes, the front sight tower is tack-welded to the barrel, to correct a slight wobble induced by roll pins fastening the tower to the barrel instead of taper pins.  Thanks all.  Ops

On my shooting glasses, I have the focal points set up for when I am mounted up.  This puts the center of lens up in the top-left hand corners (seeing out).  

The only problem is your going to need to find a doctor that will allow you to walk in with a rifle and get the glasses/lens fitted, which isn't going to happen down at your local mall.

P.S. My frames are Randolph’s that allow me to have different lens set up for different shooting sports and colors. This allows me to just pick a color of the day, and focal point for the different types of shooting (high power, trap-skeet, or sporting clays). You may want to just start off with a normal focal point in color, and clear lens set up with a mounted focal point.

Wandering zero could also possibly result from a loose barrel nut.  Easiest way to check is to put one had one the front sight and the other on the reciever and see if there is any perceptible play when you try to twist the barrel.

Practice more. I've found that in order to get good with a rifle, it's more rounds down range.