LOL, JAB you do not understand physics I see.
Aluminium foil is so thin the back EMF is next to nothing.
Get a real piece of Aluminium, something with some mass, and it will give you a good kick. I put an Aluminium washer on the face of the paddle and it shoots it nicely.
I also have a 5lb Aluminium block I use as a heat sink to cool the paddle between uses, it reacts strongly to the field.
Try Iron foil, it will do nothing.
As Beck would say, don't take my word for it, go look it up.
The back EMF depends on the amount of conductor available. Any foils, be they Iron, or Aluminium, are too small to provide any noticable effect. IE, the magnetic field on the wire in the coil by itself is nothing, but when you wind it hundreds of times, it really adds up.
Try anything aluminium with some mass, pop cans won't work, too thin, try something at least 1/4" thick. It really kicks.