We Deal in Lead is a fantastic game for recreating the Gunslinger/The Dark Tower vibes from Stephen King. Using Cairn as a base seems like a no-brainer and really adds to the Gunslinger ascetic. The author also includes a wealth of tables for exploration ideas to fuel a Warden for hundreds of adventures.

If you want a rules-lite approach to Gunslinging and exploring between the worlds then this is right up your alley.