.. when you play an ammo card like Dragon's Breath Round and the ammo card is canceled by Direct Intervention, you can play another ammo card? The reason is that while most cards (like Immortal Grapple) say "only one <card of type xyz> can be played per turn/combat/..", the ammo cards say "only one ammo card be used (on a gun)". This implies that the ammo card is successfully played, and the ammo card was played (but never resolved), you can play another one, even one of the same kind.
If Dark Influences is played on an ammo card, you can't play the same ammo card again this turn, but you can play a different kind of ammo card, of course (with the same reasoning as above).
Reference: VtES Newsgroup.