After watching a few review videos that mentioned how the HRO didn't fit anything , and had a dim dot. I had to see for myself. I took 7 different hand guards, 2 upper receivers, and even a S&W 15-22 which is listed on the site as non compatible. The HRO mounted just fine on each of them. As far as first impressions go I'm impressed. Now we have to torture test it hahaha.