Magpul Industries ACS-L Stock, Fits AR-15, Mil-Spec, Black MAG378-BLK The Magpul ACS-L (Adaptable Carbine Stock - Light) is a drop-in replacement buttstock for AR15/M4 carbines using mil-spec sized receiver extension tubes. A streamlined version of the ACS, the ACS-L utilizes the same center storage compartment and cheek weld but removes the battery tubes for a slimmer, lighter weight package.