These 8g CO2 cartridges are custom fit for the Byrna HD. Each cartridge (new) will be good for 15-20 rounds (4 mags in a 5 round mag) at full speed. Punctured cartridges within the Byrna will leak over time. It is recommended to swap out punctured CO2 cartridges and leave an un-punctured cartridge inside the Byrna HD to avoid an compressed air loss. The Byrna HD will puncture and fire the first round at the same time.