Odin Works OTR-15, Semi-automatic, Lower/Upper Set, 223 Remington/556NATO Matte Finish, Ranger Green OTR-15-RECEIVER-SET-RG The OTR-15 lower is machined in house from billet 7075 aluminum with features and style in mind. Machined to match the ODIN Works billet upper, the OTR-15 Receiver Set provides a best in class chassis. The Billet Upper Receiver has a sleek design "No Forward Assist" and low profile brass deflector. The OTR-15 Receiver Set guarantees you'll stand out in a crowd and above all else, gives you the top shooting performance that you require.