TROY Industries Battle Rail MRF, 7, Fits AR-15, Carbine Length, Drop-In, 2-Piece Quad Rail, Black Finish SRAI-MRF-D7BT-00 The Troy Drop-In Rail is a non free-floating direct replacement for all M4/AR15 Carbine length plastic hand guards 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.