After digging into this hot mess of netprops i found this
SOLUTION:
PHP Code:
int launcherEnt = GetEntPropEnt(projectileEnt, Prop_Send, "m_hLauncher");
int originalLauncherEnt = GetEntPropEnt(projectileEnt, Prop_Send, "m_hOriginalLauncher");
Both of them seems to work. Both return entity index of weapon that fired projectile.
I don't know the difference between those two so step carefully future readers.