Seinen and shounen action series are still pretty different...
Well, in alphabetical order:
*Muhyo to Roujie
*Rave Master (Groove Adventure Rave, RAVE, whatever it's called)
*Tsubasa Chronicle (Reservoir Chronicle Tsubasa)
Also, you're probably not looking for Tsuki no Shippo. It isn't really an action manga at all, and it's definitely not shounen. It's not a terrible manga, but it just really doesn't fit this thread.