The present invention relates to a gear cassette (1) for firearms, comprising: a trigger unit (3), the trigger unit comprising means for activating firing of the firearm, and a combustion unit (20) comprising: —a combustion chamber part (21) having an inlet end section (22) and an outlet end section (23) and an annular wall (24) arranged between the inlet end section and the outlet end section, the annular wall and the end sections comprising an inner surface (25) and an outer surface (26), at least one gas inlet (31, 32) and a fire outlet (33), and at least one spark plug (34) projecting into the combustion chamber, wherein the combustion unit further comprises a gas hose (6, 7) connected to the gas inlet (31, 32) of the combustion chamber part (21) for bringing the combustion chamber (74) in fluid communication with a gas unit comprising at least one gas container (4, 5), and wherein the trigger unit (3) is adapted to activate firing of the firearm by igniting gas in the combustion chamber.