TROY Industries Enhanced Rail, 7, Drop In, Picatinny on All Four Sides, Four Built-in Sockets Fits QD Swivels, Fits AR-15 Rifles, Black SRAI-DID-D7BT-00 The Troy Industries Enhanced Drop-In Quad Rail is a non free-floating direct replacement for all M4/AR-15 Carbine length plastic handguards with a round end plate. The Troy patented clamping design ensures a solid mounting platform and provides a true optical mounting platform that increases the modularity and accuracy of the M4.