Okay, I received one of the very few remaining upper receiver alloy rails that can be found for my HK USC carbine. It was made by Laser Devices and recently discontinued. I've discovered why. They advertise the piece as "to spec". Here's the weird thing....My Bushnell Holosight using a single allen wrench screw mounts and holds tight as a tick. My Aimpoint with cantilever LaRue quick detach mount does NOT hold tightly.....Why would one hold and the other be loose???