AR15 sigthing problem

[VASCAR2]

They make two heights of front sights. By looking at your rifles you put front sights on a handguard rail with a sight designed to mount on a gas block. There is a difference in height between a gas block rail and the rail on your handguard. I'd probably buy a new front sight for a railed fore end and sell the gas block sight on the equipment exchange or keep it for a later build.