If you have a 511, 512, 551 or 552 and have the 3X, the only thing that works is the Larue LT-110. But, the problem now is that you have to mount the sight much more forward than the mag to clear the QD lever, as well to clear the secondary rear riser.
Looks like this:
EoTech realized the issue and the new 4X does not have the alignment problem of the 3X.