So basically when you pull back the second bottle you get suck in air into the "Chamber", and when you wack it forward you push air down the barrel? I like
I bet if you could some how seal the chamber (so theres no air leaking out the sides of the bottles when you move it forward) it'd go much faster.
Oh yeah, and maybe super big rubber bands that go from the top of the bottle to the bottom? This way you can juts hold it back and let go and it fires lol.