Hammer head assembly used in an air nailing gun for driving U-nails

A hammer head assembly installed in an air nailing gun to guide a striking rod of the air nailing gun to drive a U-nail into a wooden material, comprising:

• an externally threaded, stepped, hollow, cylindrical mounting base fastened to the air nailing gun, said mounting base having a center through hole axially extended through top and bottom ends thereof for the passing of the striking rod of said air nailing gun;
• a sliding sleeve inserted into the center through hole of said mounting base and partially extended out of the bottom end of said mounting base, said sliding sleeve having a center through hole axially extended through top and bottom ends thereof for the passing of the striking rod of said air nailing gun, and an outward stop flange extended from the periphery of the bottom end thereof;
• a first spring member mounted in said mounting base and adapted to impart a forward pressure to said sliding sleeve;
• a coupling nut sleeved onto said sliding sleeve and stopped above the outward stop flange of said sliding sleeve outside said mounting base, said coupling nut having an inner thread;
• a nail holder adapted to hold a U-nail for fastening, said nail holder having an outer thread threaded into the inner thread of said coupling nut, a flat receiving slot extended to a bottom side thereof and adapted to receive a U-nail for fastening, and a top insertion hole disposed in a top side thereof in communication with said flat receiving slot;
• a hammer rod mounted in said sliding sleeve and aimed at the top insertion hole of said nail holder for driving by the striking rod of said air nailing gun to force a loaded U-nail out of said nail holder into a wooden material;
• and
• a second spring member mounted inside said sliding sleeve and adapted to push said hammer rod toward the striking rod of said air nailing gun.