Glad somebody tried it!
I threw this idea out a while ago - to less than enthusiastic response!

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=117620&an=0&page=1#Post117620

I thought the 1/2" caps in a 10ga cup would be what I would try.

Are you loading them with the lead or theopper facing forward?