Walker served in the Vietnam War with Frank Castle (Punisher) Now he seems to work for an unnamed federal law enforcement agency so secret that even he doesn't know its true name(whatever it is, it evidently has very deep pockets- as he says that nobody ever asks to see his expenses vouchers).
Shotgun was created by Ann Nocenti and John Romita Jr in 1989 and first appeared in Daredevil # 272.
Major Story Arcs
Daredevil: Lone Stranger
For more information see: Lone Stranger
Shotgun is hired by Skip Ash to bring Number Nine back to his farm. As part of the payment Shotgun is give a shotgun that shoots explosive rounds. He tracks Number Nine to Skip's daughter, Brandy's farm. There he is able to ambush Daredevil, who had helped Brandy earlier.
Daredevil beats up Shotgun and sends him packing back to Skip. Skip then rearms Shotgun with a better gun and tank like car. Shotgun comes back to the farm but this time Daredevil is ready for him and is able to destroy Shotgun's new gun and car.
Daredevil leaves Shotgun trapped under his car, hurt but alive.
Bullseye: The Colombian Connection
For more information see: Bullseye: The Colombian Connection
Shotgun was hired by agent Joy(Shadow Tracer) from the FBI to help her track down Bullseye. He was revoked of his citizenship because of his mercenary actions. Joy offered him 1,000,000$ and the legal documents to return to the USA if he would help agent Joy in this acquisition. He demanded more but his partner - Bullet, offered him half of his share and he accepted at the end.
Weapons & Abilities
Shotgun is a highly trained operative and marksman. Shotgun has been equipped with a number of high-tech weaponry including a specialized scoped shotgun that fired explosive, concussive, combustible and disintegrative rounds in a rapid succession. Shotgun carried numerous firearms including a pistol hidden in his wrist gauntlet. Shotgun had at least two ammo belts on him and he wore glove/arm sleeves with a row of tiny spikes on them for close quarter combat.