An extension device (1) of a firearm (500) mounts in an operating configuration, to a muzzle (555) of a barrel (550) of the firearm (500). The extension device (1) includes a main body (10) which has an inner element (11) housable inside the barrel (550) and suitable to firmly engage with an inner barrel wall (550′) and an outer element (12) which extends axially adjacent to the inner element (11) and is suitable to be arranged outside the barrel (550) with the inner element (11) housed in the barrel (550). The extension device further includes a secondary body (20) fitted axially on the main body (10) and an attachment body (30) fitted axially on the outer element (12) and fixable thereto performing an axial thrust action on the secondary body (20) until the secondary body presses against the muzzle (555) of the firearm (500).