
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.