I took the barrel off of a Colt 6530 to put it onto a flat top upper. Long story short, in two flat tops, the sights had to be moved way to the left in order to get zeroed. I ran out of room to sight in on a third upper I put the barrel on. The original A2 upper the barrel sat in zeros just fine, maybe 3-4 clicks of windage and it's dead on, even after returning the barrel to the original A2 upper. The barrel was a Colt factory job and had never been taken off before I did it.
I guess my question is, would Colt have filed and pinned this barrel into place at the factory? This would mean the front sight and the barrel extension aren't lined up, correct? Could I have done something wrong with the three different flat tops I put the barrel on? I have changed/swapped barrels probably 40-50 times, and am not a gunsmith by any means, but I've never had a problem with excessive windage before. If a gunsmith can easily correct this either by removing the barrel extension or file and peening into the new upper, I'll have this done. If it's not easy, reckon I'll sale the original complete upper to somebody that wants an A2. Thanks.....Chad

I have a gut feeling that you bent the barrel extension pin when you re-installed the barrel on a new upper.

Pull the barrel, and check to see if the pin is in line with the front sight block, or if it's bent.  If the barrel was fine on the colt upper, it should have been fine on the uppers.

Also, after you install the barrel, you will need to fine-tune the barrel index to the upper by live fire to get the rear sight centered.  A few taps on the front sight with a leather mallet will shift the barrel in the upper to allow the rear sight to be centered at zero.

Thanks Dano. I too thought I had bent the pin. So I took the barrel of and looked; I couldn't "see" it. Because of this, I put the barrel on it's original A2 upper. It zeroed fine, just like it had before (I don't remember how much I had to adjust the windage, it wasn't much, like 3 or 4 clicks).
I'll take it off again tomorrow and look at it with a level or straight edge. I'll put it back on the original upper and zero it again. If all is well, I'll try the mallet whack when mounted on the flattop.  
Darn thing has me miffed.
Thanks.....Chad

When you have the barrel off, look straight down the top of the barrel to make sure that the index pin and front sight are indexed on the same plain.  If they are, then check the flat top receiver barrel index grove, you may have enlarged the slot if you over tightened the barrel.  If needed, shim the pin in the upper to keep the barrel sight indexed when you tighten the barrel.  It sounds like the barrel/receiver is slipping when you are tighten the barrel (real problem if you use a barrel vise block, and not a upper vise block).

As for a completely centered rear sight zero, it will need to be done by live fire, using the leather mallet to drift the barrel in the upper receiver (re-index the front sight to the rear sight by drifting the complete barrel in the receiver, not just the front sight on the barrel).

CRS188,

The alignment notch in your flattop upper may be a little wider or it may be slightly mislocated. Either way you should be able to install the barrel and then drift the barrel over into alignment.

If you install the barrel per the TM you shouldn't have to do this but I prefer just screwing them on, going to the range, and adjusting from there.