The box of ten 8g CO2 cartridges are custom fit for the Byrna HD. Now including 1 CO2 cartridge with oil for simple maintenance every 10 CO2 cartridges used. Each cartridge (new) will be good for 15-20 rounds (4 mags in a 5 round magazine) at full speed. Punctured cartridges within the Byrna HD will leak over time.