The same racks are in use today, (M12 series rack). Nowadays they are modified to fit, first the M4 carbine and now are being modified for the ambi, M4 carbines. Kind of strange that the nomenclature is the M12 rack, instead of perhaps the M16icon rack, but that is the military. The modification is a simple angle iron welded to the back of the rack's handguard rest to prevent the M4 from being removed while locked. For the M4A1, they simply bend the handguard rests to accept the different width. These racks are often for sale at the DRMO sales. Usually bought for scrap.