How many weapon fragments do I receive from killing an alien?

I want to maximize the number of weapon fragments I salvage, but sometimes bad dudes just need to be blown up to save my team. If there are differences in the amount of fragments I can salvage from different enemies or different weapon types, I can use that information to determine which enemies to blow to smithereens, and which to kill with normal weapons fire.

For each enemy I kill with normal weapons fire, what determines the amount of weapon fragments I receive at the end of the mission?

• Is it just a set number of fragments per weapon?
• Does it change depending on the enemy type or their weapon type?
• Do robotic aliens with built-in weapons still provide weapon fragments, or do I only get them from enemies with hand-held weapons?

Each weapon gives 2 weapon fragments. Here are some rules regarding what will drop weapon fragments (WF):

• An alien killed by rifles, pistol fire, psi attacks, or melee weapons will always give WF.
• An alien killed by grenades, rockets, exploding cars, or exploding mechanical aliens will not give WF.
• Some enemies (such as Mutons, Heavy Floaters, and Cyberdiscs) carry grenades which are also worth an additional 2 WF, if they are killed before they use them.
• If an alien is killed while mind controlled or captured, its weapons will not become WF, but instead will be delivered home whole.
• Chryssalids, Ethereals, and Zombies don't carry weapons, so they won't give WF no matter how they're killed.
• Machines (Cyberdiscs, Drones and Sectopods) give 2 WF when killed, unless killed by explosives.

So, for example:

• A Cyberdisc or Muton will drop 4 WF if they have not used their grenade, or 2 WF if they have.
• A Drone or a Thin Man will drop 2 WF.