The TopStop cover is designed to snap on most AR15 or M4 type upper receivers when they are detached from the lower receiver.  The TopStop completely seals the upper receiver preventing fouling from getting in or out and it stops the bolt carrier group and charging handle from sliding out as the upper is moved around.  The TopStop is made out of super tough polymer and is impervious to most solvents and chemicals.  This product is perfect for storing or transporting your spare upper recievers, especially in low-profile bags where the rifle must be broken down to fit.