7" Carbine/M4 Drop-In Rail. The Troy Drop-In Rail is a non free floating direct replacement for all M4/AR15 carbine length plastic hand guards. The Troy Patented clamping design ensures a solid mounting platform. The Drop-In provides a true optical mounting platform that increases the modularity and accuracy of the M4. No gunsmithing is required and the unit installs in less than a minute. Provides true U.S. Military Specification M1913 rails at 12-3-6 and 9 o'clock. All rail systems are machined from T6061 hardened aircraft aluminum and finished in MIL-STD Type III Hard Coat Anodizing.