Ok I was looking up cannonball on this site and it says he died 2 times http://comicvine.gamespot.com/x-force-7-under-the-knife/4000-35378/ and The New Mutants #37 - If I Should Die but it dose not say how he died. Plus how did he comeback to life ?
If I recall correctly re: X-Force 7 it was revealed (by Cable) he was an External aka basically immortal (later retconned-ish? / ignored by subsequent creative teams, and for an immortal group of mutants they have a spectacular ability to stay dead). New Mutants #37 the whole team "dies" during Secret Wars II, Beyonder kills them for kicks & subsequently resurrects them.