Updated to include The Scarlet Keys Player Expansion cards:

Directly:

• Ever Vigilant (and now Ever Vigilant lvl 4)
• Prepared for the Worst [lvl 2]
• Sleight of Hand
• "You owe me one!"
• Untimely Transaction
• Friends in Low Places (after taking the Swift upgrade).
• "I'll take that!"
• Salvage

Indirectly:

• Swift Reflexes
• Honed Instinct
• Dexter's ability.
• Joey "The Rat" Vigil.

There, someone should have catalogued Old Shotgun's enablers ages ago. Thank you to Until the End of Time for finding all of them.

Edit: Added Joey as per Django and Yenreb's insights. And I'll elaborate on how Dexter and Joey's fast playing of items allows Old Shotgun to enter play with ammo:

Step 1.) Play an event with a skill test (Backstab, Ríastrad, Clean Them Out, etc...).

Step 2.) During the skill test while resolving this event, you get a window of opportunity. Use this window of opportunity to trigger a fast play action with either Dexter or Joey.

Step 3.) You have technically played Old Shotgun while playing an event. Thus, is comes into play with its uses value as 2. Voila.

So the interesting thing about this card is obviously the clause that you won't find on any other card (so far), and that is "While playing an event, treat Old Shotgun's uses value as 2." This is easy to misunderstand (I know I did initially), so I figured I'd write a short little thing to clarify this for anyone who happens to be confused.

The main thing to remember about this effect is that the main role of Uses is not to determine how much ammo your gun currently has, but rather how much ammo it enters play with. This means that playing events after your Old Shotgun is already in play (such as Marksmanship or "Eat lead!") won't reload it automatically.

So, for that clause to matter, you want to be playing an event while it's entering play. The most obvious way of doing this is by playing an event which is supposed to put an asset into play, such as Ever Vigilant and Sleight of Hand. However, you could also try to get the card into play while playing an unrelated event, such as an event with a skill check, giving you a window to trigger fast abilities that put assets into play such as with Dexter Drake's ability and Joey "The Rat" Vigil. Swift Reflexes might work for this too, though I'm not sure on this one. Once it's entered play, the only event which still cares is Swift Reload, which explicitly checks the Uses value of the card (and so will reload it back to 2.)

This probably won't be a weapon you just happen to pick up - to really get the value out of it, your deck will need to have plenty of support for it (ways to get it into play with actual ammo and the usual ways of reloading guns afterwards, on top of wanting to oversucceed on the fight action). If you do though, you'll have access to a 0R 2XP gun with a pretty good fight bonus and (potential) damage that no other weapon at this XP level can compete with. Is that worth all the effort? Hard to say, but because of how weird this card is, I'm sure plenty of people are gonna try anyway.

Edit: Since making this review I've thought of a few more examples of ways to get it into play with ammo for those with survivor access, which are Scavenging (2) during an Investigation event, or Yorick using his ability during a Fight event where he defeated an enemy. It might inspire someone to make an Old Shotgun deck around that idea, so I thought I should add it here.