Re: xp clone action?
Once a rifle, always a rifle. A rifle has a barrel over 16.5" long and an overall barreled action of at least 26".
HOWEVER, if you buy an undesignated action, that is an action that has not been anything before, you can make it anything you want.
I hope this answers your question.
An making a center grip pistol is mostly making a trigger mount with linkage. Personally, I would make it electric so less to fool with. Say using a paintball gun electric trigger setup?
Unless you're real good with machining. Reinventing the Savage or Remington linkage is gonna be a little involved.