Hera USA HRS Buttstock, Fits Full Length AR-15 A2 Buffer Tube, Black 12.40 The Hera Arms HRS fixed buttstock is an easy-to-install replacement made from fiberglass-reinforced polymer. Also features removable covers that provide additional storage capabilities. The HRS stock is designed to be used with AR-15/M4 systems featuring a standard A2 (rifle) buffer tube. The HRS can also be used on any rifle system that makes use of an A2 rifle-tube.