ガンシスター

250 years from now. Human race is reduced to 2.5 billion due to fierce confrontation with the fairy race. Many secrets of lost civilizations are lost in the chaos.
Main character is a travelling gunman. One day he gets into bitter rain and regains consciousness at a monastery with his right arm and left foot injured. He starts to live there with a priest and five sisters and gradually gets involved into the battle they lead.

[From vndbreview]