Is it the puncture tool with the little folding handle that magnetically retracts into the screw?
Screw it all the way in, until it falls out into the area where you'd put the 12 gram.
If it's the First Strike compatible magazine with the plastic knob that magnetically retracts into the screw, there's a small pin that holds it on the screw. Knock it out with a small allen wrench, pin punch, or nail. Then screw it in until it falls out into the area where the 12 gram goes.
David Johnson, AKA Fubarius.
Nunquam emere potes facere