I have spent a week now looking for this manga of a boy at a carnival or some kind of festival. He finds a game tent with a woman inside who explains that its a water gun game, but then he finds out that she is the target. He shoots her and gets her soaking, then has sex with her and she gives him a picture as his prize.

Any ideas?

P.S. I think i remember the book saying the woman was older or mature if that helps...