An effective bum gun sprays water with a lot of force. There's a certain way to aim it (to be specific, first near your anus and then circle outwards) that completely removes all feces in just a few seconds.

I have tested the cleanliness with paper, but after 6 years now I never need any paper. Unless your ass is really hairy there won't be as much residual wetness as you'd expect and air drying for a moment is enough.