An apparatus for the section-wise autofrettage of gun barrels (22) by a hydraulic pressure generated inside the barrel. The apparatus (1, 1′, 1″) comprises a mandrel-shaped insert (2, 2′, 2″) inserted into the barrel (22) from the breech opening. A first sealing device (3) is disposed at one end of the insert (2, 2′, 2″), and has a seal packet (5) disposed on a seal seat (4) of the insert (2, 2′, 2″). The seal seat (4) is limited at the front by a threaded sleeve (7) screwed onto the insert (2, 2′, 2″), and an annular intermediate part (8) adjoining the threaded sleeve (7) at the rear. The seal packet (5) includes at least one O-ring (10) held by a shoulder (9) of the mandrel-shaped insert (2, 2′, 2″), and a high-pressure seal (15) formed by two partial rings (13, 14) that are connected to one another by a conical contact surface (16).