Currently I'm experimenting using EEZOX for both interior and exterior damascus-barreled guns, most of which are black powder percussions. It dries completely so there's no danger of seepage.

On a different BP oriented board it came out on top along with Ballistol as the best rust protectant for long term storage.

As I mentioned earlier, Renaissance wax has been my main rust inhibitor and still is on fluid steel-barreled guns, as well as some other damascus barreled guns, truth be told.