CO2 Mag for GBBR
 

Just wonder, do I have to finish off whole CO2 canister before I can take it out? OR I can take it out even I haven't finished the whole canister?
I worry the canister will fly off once I unscrew the cap.

Run it to the end, don't vent it as it's not good for the seals, open slowly so it'll seep out slowly, then contnue unscrewing it once it stops hissing

It won't fly out, it'll vent before you can unscrew it far enough. As Pus mentioned, it's not that good for the seals so let out slowly. I had zero problems just venting but that was with an air rifle.