The flashbang, or M84 stun grenade, is a perforated steel casing wrapped around a magnesium core. When detonated, the fuse/grenade body assembly remains intact. The body is a tube with holes along the sides that emit the light and sound of the explosion. The explosion does not cause shrapnel injury, but can still burn. Theconcussive blast of the detonation can injure and the heat created can ignite flammable materials such as fuel.
tl;dr flashbangs are metal tubes filled with magnesium, and that shit burns.
The same applies for Willy Pete or White Phosphorous grenades in Black Ops, although those are considerably “nerfed” compared to their real-life counterparts. If you think Willy Pete is a funny name, you want to try reading up on its effects on people some time.
There is no such thing as non-lethal ammunition. The human body is a lot more fragile than you think.