Normally said when people are starting to get into a scuffle and/or fight . It can also be said when someone throws something , there is a loud noise ( which should be accompanied by ducking ) and in the most extreme case , literally.
1.Ishmael head a huge bang from the back fo the room. He screamed " They shootin' " and ducked under his desk.
2. " They shootin" Said Steve looking at the two girls fighting in the front of the room.