Reddit reviews The Great Book of Guns: An Illustrated History of Military, Sporting, and Antique Firearms

Hopefully you've tried contacting your target to ask them about their collections, their uses, stuff they want, and stuff they wouldn't like. Feel free to drop as many Phone Booth references as possible. You should be able to get some ideas.

If you can't find anything specific, the stuff you've mentioned will never go amiss. Some other stuff I could think of:

• Targets, be they paper, adhesive, or metal
  
  
• Gun bags, belts, storage and transportation solutions, either generic or tailored to what you know of their collection
  
  
• Magazines (specific to their gun and of legal capacity; you can never have too many)
  
  
• Some ear or eye pro
  
  
• Gift certificates or range memberships for businesses in their area
  
  
• Action movies with gun porn (anything with a long-ass entry on IMFDB)
  
  
• A nice coffee table book
  
  
• Generic tools, like a pocket knife, multitool, or tacticool flashlight
  
  
• If they're a gun-toting gamer, stalk their Steam/Xbox Live/Playstation accounts and pick out a game they'd like (reloaders will particularly enjoy Fallout New Vegas)
  
  
• Beef jerky

Throw this at them:

http://www.amazon.com/gp/aw/d/159223304X

I don't think he's got any guns like the AR-15, just 1-2 hunting rifles and a lot of handguns. Just a cursory Amazon search gives me some good starting points. This might be good for the history of handguns or maybe a more general interest history, like this one.