This new upper receiver is completely compatible with your M16 (small front pin) lower receiver and allows the user to change barrels in just seconds. This allows the system to be extremely compact and concealable as well as giving the operator the option of having multiple barrels for multiple functions. The best part of our QCB Upper Receiver is that it utilizes standard M16 barrels! Take a close look at the photos and you will see that the barrel is a standard M16 barrel with the Delta Ring, barrel nut, sling swivel and front handguard keeper removed. That's right, you can use standard barrels just by removing the items mentioned above! The New QCB-D model now is compatible with piston systems!!! The rails have also been changed to create less sharp edges against the shooter's hand as well as creating cleaner lines and appearance. This is the latest revision with multiple improvements such as Hard Coat Anodized Mil Spec Finish, four full-length Picatinny style rails, new locking arm retention system and more. * BUIS (Back Up Iron Sight), Charging handle, bolt and carrier group not included with upper receiver.