It's a sight hight issue, if your railed gas block was at the same level as your forearm rail then it wouldn't be a problem. If that were the case one might as well mount it to the forearm rail. Most often than not the gas block is lower than the forearm rail. The sights were designed for the front sight to mount on the rail. If you mounted the front sight on the gas block rail. Chances are it would be lower and there would be issues with "Angle" and sighing problems. The MBUS sights are just a emergency back up. I have several sets I use but purchase quality aluminum or steel sights for my serious carbines.