[Jonny250]

OK, here's my theory, bear in mind that i know NOTHING about ballistics, etc, but am trying to apply a couple of physics lessons... perhaps someone else can check/correct it as necessary...

From Hooke's Law:
F = -K.x

Elastic energy:
E = (K.x.x/[2.L])
K = elastic modulus
x = extension of band
L = original length

k should be the same for bands of the same material, not related to thickness of the band.
x and L can be measured.

This will give the amount of energy for each system, dont forget to apply this to each band used.

You then need to look at the momentum of the shaft and equate the energy in the system = 1/2 .m.v.v
This should tell you the velocity of the spear leaving the bands.
m is the mass of the spear and will be a function of the length and diameter.
You could use this to get an idea of equivalence between bands and guns, however i dont know if the speed of the shaft leaving the band is directly related to the distance it can travel, but i think so.

I hope someone else can add to or correct this...