AR Barrel Indexing

[Guest nosnos]

I finally got my first AR to the point where I could take it out and shoot it. At 25 yards I started sighting it in, and found I had to crank my rear sight almost all the way to the left to get the windage right. Once I got it there it stayed pretty good out to 100 yds, which is as far as I really plan on shooting anyway. My question is, what could be causing such alignment problems that my rear sight has to be all the way to the left?

I recently put on a free-float rail (thanks again TNRoboCop) and it seems to be lined up well enough. I hadn't shot the gun before that, so I don't really think the rail changed anything, but am not quite sure. The front sight is on the rail.

I've thought before that looking down the upper it seems like the barrel drifts slightly to the right, and it seems that I've read there can be "indexing" problems with uppers sometimes. Is that possible? What can I do to help this? I'm just OCD enough to not want my rear sight being cranked all the way left. Any input would be greatly appreciated...

[TNRobocop]

The barrels are indexed from the factory via the barrel extension. Very rarely are they "out of whack". It is possible the upper receiver extension slot might have been cut off to the side but hard to tell.

Shoot it and see before anything else. Our military guns sometimes have windage and elevation adjustments that seem out of whack, but it still zeroes.

If you have problems after zeroing it, bring it back over and we can do some more work on it.