This is a Japanese children’s prank involving poking someone’s bum (anus) by clasping their hands with their index fingers pointing upwards like a gun. Typically, you can see kids do this among close friends around the ages between elementary school.
Mahjong is a table game that originated in China. Generally four people play against each other, but there are many cases where the game only includes three players, especially in the Kansai area. Japanese people learn Mahjong when they are high school or university students. However, only about 30% of students can play Mahjong nowadays.