I've got a leupold vari X III mounted on a free float rail upper, and at max adjustment, I'm still over a foot to the left at 100y.  The same scope on an M1-A shoots just fine with the windage very much near mechanical zero.

I had an IOR M2 on it, and that was fine.  It's only this loopy with the 1" tube that's givin' me grief.

I'm gonna try reversing the rings, and maybe bring them inboard torward the turret more.

Is there any other remedy for this?

--Fargo007

It certainly sounds like a mounting problem.

What rings and bases are you using?

viator

It's a predator rail, which slides right over the upper receiver.  Warne medium tactical rings on top of that.  Everything locks up correctly and tight.

--Fargo007

Remount and try again.

Try another set of rings, too. You just never know.

definately a mounting issue

Since the scope has proved to be centered in another application, and assuming that it has not been inadvertently damaged in the interim, mounting must be the problem.

Have you had another scope on this flattop? If you have and it worked, you know that the flattop is probably within manufacturing tolerances.

Have you used the rings or predator in another application? Did they function well?

Do you have a ring alignment tool to ensure that there is no twist on the scope tube?

Your problem probably lies in one or more of the following areas:

1) the machining of the rails on your flattop;

2) the predator; or,

3) the rings.

I don’t think that it is likely for production tolerances to stack up to such an extent that your scope would be that far off.

viator

  Before you remount make sure you zero your windage and elevation.
Turn knob all the way to left or right. Then turn all the way the other direction, counting the number of turns or clicks (clicks is more exact but difficult) then return exactly half the distance back the other way. This leaves you with optimum adjustment in either direction. Repeat with elevation, then remount.
  Always do this when mounting a used scope, as adjustment on previous weapon leaves you off center. I've also seen factory new scopes that were not centered
  I have been accused of being too detail oriented.