A gas cylinder actuator (10, 110, 210, 310, 410) with safety device for uncontrolled return of the piston-stem, which comprises: - a tubular containment jacket (11, 111, 211, 311, 411), - two opposing heads (12, 13, 412, 413) for closing said tubular jacket (11, 111, 211, 311, 411), a first head (12, 412) which is provided with a through hole (14, 414) for passage and for translation guidance with respect to an axis of symmetry for a piston-stem (15, 115, 215, 315, 415), and a second, opposing head (13, 413), - a piston-stem (15, 115, 215, 315, 415), which comprises a stem portion (16, 116, 216, 316, 416) and a piston portion (17, 117, 217, 317, 417), - sealing means (23, 123, 223, 323, 423) which are arranged so as to act against said stem portion (16, 116, 216, 316, 416) at said passage and guidance hole (14, 414), - a chamber for pressurized gas (18, 418) being defined between said tubular jacket (11, 111, 211, 311, 411), said heads (12, 13, 412, 413) and said piston-stem (15, 115, 215, 315, 415), said gas cylinder actuator (10, 110, 210, 310, 410) further comprising, between said first head (12, 412) and said piston portion (17, 117, 217, 317, 417), a bushing (20, 120, 220, 320, 420), which is coupled to said tubular jacket (11, 111, 211, 311, 411) with means of preventing translation with respect to said axis, and is designed to encounter said piston portion (17, 117, 217, 317, 417), said bushing (20, 120, 220, 320, 420) being configured to at least be deformed in the event of impact with said piston portion (17, 117, 217, 317, 417) owing to the uncontrolled return of said piston-stem (15, 115, 215, 315, 415), so as to allow the passage of said piston portion (17, 117, 217, 317, 417) and the formation of a gas discharge passage between the piston-stem (15, 115, 215, 315, 415), the sealing means (23, 123, 223, 323, 423) and the passage and guidance hole (14, 414), characterized in that the sealing means (23, 123, 223, 323, 423) are constituted by an annular gasket which is pressed between an outer surface (37, 437) of the stem portion (16, 116, 216, 316, 416) and a facing portion of the internal surface (38, 438) of the jacket (11, 111, 211, 311, 411). - a tubular containment jacket (11, 111, 211, 311, 411), - two opposing heads (12, 13, 412, 413) for closing said tubular jacket (11, 111, 211, 311, 411), a first head (12, 412) which is provided with a through hole (14, 414) for passage and for translation guidance with respect to an axis of symmetry for a piston-stem (15, 115, 215, 315, 415), and a second, opposing head (13, 413), - a piston-stem (15, 115, 215, 315, 415), which comprises a stem portion (16, 116, 216, 316, 416) and a piston portion (17, 117, 217, 317, 417), - sealing means (23, 123, 223, 323, 423) which are arranged so as to act against said stem portion (16, 116, 216, 316, 416) at said passage and guidance hole (14, 414), - a chamber for pressurized gas (18, 418) being defined between said tubular jacket (11, 111, 211, 311, 411), said heads (12, 13, 412, 413) and said piston-stem (15, 115, 215, 315, 415), said gas cylinder actuator (10, 110, 210, 310, 410) further comprising, between said first head (12, 412) and said piston portion (17, 117, 217, 317, 417), a bushing (20, 120, 220, 320, 420), which is coupled to said tubular jacket (11, 111, 211, 311, 411) with means of preventing translation with respect to said axis, and is designed to encounter said piston portion (17, 117, 217, 317, 417), said bushing (20, 120, 220, 320, 420) being configured to at least be deformed in the event of impact with said piston portion (17, 117, 217, 317, 417) owing to the uncontrolled return of said piston-stem (15, 115, 215, 315, 415), so as to allow the passage of said piston portion (17, 117, 217, 317, 417) and the formation of a gas discharge passage between the piston-stem (15, 115, 215, 315, 415), the sealing means (23, 123, 223, 323, 423) and the passage and guidance hole (14, 414), characterized in that the sealing means (23, 123, 223, 323, 423) are constituted by an annular gasket which is pressed between an outer surface (37, 437) of the stem portion (16, 116, 216, 316, 416) and a facing portion of the internal surface (38, 438) of the jacket (11, 111, 211, 311, 411).