Ok, are these all supposed to be interchangeable? I have a Troy Alpha handguard that came with a rail that has a QD socket in it. I tried a buddies QD sling mount and it would not lock into place. Are they each proprietary upon themselves or is one of these parts faulty? His UTG DID lock into a UTG stock. I dont want to order a Magpul QD only to find out it wont do what I want.