Ok, I've now tried two different uppers (M&P carbine and PSA mid-length) and both require the rear Magpul sight to be moved over more than half way to the left. Originally I figured my M&P FSB was canted but now I'm not sure so sure. I don't have another rear sight to try. Not a huge deal and I zeroed my new PSA upper just fine but I am a bit OCD. Wanna kill these ads? We can help!