An end plate is a small steel plate that sits between the lower receiver and castle nut on AR-15/AR-10 carbines. It helps secure the buffer tube (receiver extension) and retains the rear takedown pin spring/detent; many versions add sling attachment points (QD or loops). Works with: AR-15/AR-10 lower receivers, carbine buffer tubes (mil-spec or commercial), castle nuts, buffer and spring, and collapsible stocks. Essential for reliable stock installation and proper retention of the takedown pin hardware.