The TopStop cover is designed tosnap on most AR15 or M4 type upper receivers when they are detached from the lower receiver. TheTopStop 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. TheTopStop 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. Specifications for Raven Concealment TopStop Upper Receiver Cover: Manufacturer: Raven Concealment Fabric/Material: Polymer Quantity: 1 Gun Model: M4, AR-15 Package Contents: Raven Concealment TopStop Upper Receiver Cover