Also, since the mag is much thinner than a standard 9mm mag, it may fall out of a 9mm mag pouch (this has happened to me more than once as well) unless you have a special pouch for these thin mags or adapt your mag pouch to these thin mags. I once carried a friend's mag (that I was borrowing) and a few 12g CO2's in the mag pouches on my dropleg holster, and my mag in my gun. Over the course of that day, my mag fell out of my gun (fortunately I found it later), his mag fell out of my pouch (fortunately someone else found it right away), and I lost 2 of 4 CO2 containers I had in my pouches.
In short, when you put anything smallish in a pouch, especially on a holster, shake it upside down and see if it falls out. If it can fall out, it will.
I actually don't use mine much either, so I don't carry extra CO2 anymore. I'm screwed on the clip though, because it'll fall out whether it's in a pouch or in the gun.