To charge the Byrna, first assure sure that you’re using the appropriate 8 gram or 12 gram Byrna CO2 cartridge. Unscrew the CO2 end cap utilizing the hex head on the magazine base plate if necessary. Insert the Byrna CO2 cartridge into the chamber, neck first, and replace the CO2 endcap. When screwing the endcap back in place, use the hex head to tighten the endcap. Once you feel resistance, continue turning for another quarter turn. Take care to not over tighten the CO2 endcap.
At this point the Byrna is charged, but the CO2 remains unpierced allowing it to be carried in this manner almost indefinitely.
From this state, to fire the Byrna simply switch the safety from safe to fire and pull the trigger. The first pull of the trigger will simultaneously pierce the CO2 cartridge and fire the first round.