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.Features :Designed to Snap on Most AR15 or M4 Type Upper ReceiversTough Polymer ConstructionWill Only Fit Uppers that Are Detached From the Lower