Eldeguello ,
I' ve used nothing but Bore Butter for lubing and storage since I started muzzleloading , and I' ve never had any trouble with rust . Is it possible that your ML was simply stored in a place with high moisture ? Did you take any other precautions to avoid rust such as keeping the gun in a silicone impregnated sock ?
Well, the rifle was on a rack over the fireplace mantel, here in S. PA, where the air is always full of moisture. It' s not like New Mexico here! But probably no worse than where you are there in IN. No, I didn' t have it in a gun sock, but I now keep ALL my guns in these things!
Now, I' ve never had to swab between shots when I use Pyrodex, either. But always have to when using real black powder!