Hi Alex,
Seems like a good night for stuffing caps
I've been using a slight variation of the technique you used. I open up the can and removing the steel ring like you've done. Then, I heat up the can with the wafer and pins in place. A slow, steady twist & pull with pliers on the pins and viola the whole slug comes out. Finally, I unrolled the aluminum and goop to free up the pins. This way the entire base is in great condition.
I like Eric's technique too. A pipe cutter can make quick work of the aluminum can. It's especially nice when the cap is in a cardboard sleeve.
Good luck,
Bob