The invention comprises a folding stock (17) used in
carbine kits that convert short-barrelled firearms (22), featuring a folding stock (17) that enables the firearm (22) to be attached to the
carbine kit in a single motion without prior preparation on the firearm (22) or the
carbine kit, characterised in that the hinge (1) is positioned on the firearm slot (21), a hinge pin (4) centred in the pin socket (1A) of the aforementioned hinge (1), a hinge spring (6) centred on the aforementioned hinge pin (4), and a locking tongue (9) that locks the aforementioned folding buttstock (17) in the open and closed positions.