I just completed my build and began putting my rear BUIS on the upper.  Problem!  The sights won't slide on to the rail.  As stated in title, sight is Troy di-optic fixed BattleSight.  Upper is Aero Precision (blem).  It seems like the coating on the parts it maybe a smidgen too thick.  I was able to get the Troy going, but then it stopped.  I pulled it off and tried again, a few times.  Each subsequent installation seemed to move a bit farther if I worked the site based side-to-side.  I worked at it for some time, and got the site just past the first rail slot, but not far enough to put in the pin.

I have sent an email to Troy asking about fit issues and how to verify if I got the "real deal".  I did purchase from reputable on-line retailer, so I don't think it is a knock-off, but would like to be sure.  If all checks out with Troy, I will likely resort to some light filing as research shows that is not unheard of and the friction wear I was inducing seemed to be working, but it was too much effort to continue in that manner.

Any thoughts from the hive?

I'd probably suspect the issue is with a blemishes part first. It could also just be a tight fitting assembly.

Thanks.  I am in contact with Troy to make sure I have the "real deal", if I do, then I will do some light filing on the sight to make a better fit.  I will do it very slow just a few passes at a time so I don't wear through the coating.  Although it is an aluminum part so that really should not be an issue if it does occur.

I thought troy was stanag NATO  mount instead of picatinny, lots of people have issues. Mostly the angles on the clamps aren't the same. I could be wrong on this.

This.. and F troy

You guys were right on the money.  The upper left angle on the mount base was not steep enough.  I ended up taking a triangle file to it and it fits now.  I will use like this until I get an optic and replace with a folding rear sight.