Where is the magazine leaking from? If you don't know for sure, grab a lighter, and run the flame around all the sealing surfaces. Once you find the spot(s), you'll be able to figure out how to take care of it from there.
It is the o-ring, but you need to find out which one of the 6+ o-rings it is (knocker valve, base plate, or fill valve o-rings?). If it's coming from between the base plate, and the magazine, pop the retainer pins, and go to Ace, then find an o-ring that matches closely, and replace it.
The fill valve/knocker valve rings are more difficult, because they have to be metric (slight size difference between imperial and metric makes a big difference in whether or not it works.)