Possible Clues:

• Individual's passionate on a particular occasion
• Nonrecurring publication
• TV special, e.g.
• Pistol dueler's allotment, usually
• Like a deal that won't happen again
• It isn't repeated
• Isolated appearance
• Two-bore gun more than adequate for this attempt?
• No returns with those turning out to be this?
• Accomplished with a single try