~NATIVE mods that allow different missions,
~ online hacks,
~free mode hacks,
too many examples to type.
editing the gun specifics, like bullet time, damage, physics, properties of each handleable gun,
its reload time, how many bullets in the clip,
then you can go indepth and make shot guns that shoot grenades, or a fully auto mp5 shooting molotov's, or an mp5 shooting rpg's at every second.