Vibranium shield designed and manufactured by T'Challa originally for Steve Rogers after he resigned as Captain America, and took the identity as The Captain. When John Walker became U.S. Agent he used the costume that Rogers had worn as The Captain and his vibranium shield.
More recently, the shield sporting a black center ringed by a white circle and then that by a red circle was the property of April Manning, a dangerously radicalized super-soldier who fought U.S.Agent while using that shield. John would defeat him, with some help, and claim the shield as his own.