What is the optimum amount of rubbing alcohol for explosions?

I am working on a mini-cannon based project. The cannon has a chamber with interior volume of 15.2 cubic inches. For propellant I am using rubbing alcohol (isopropyl alcohol) that is electronically ignited. The barrel on this gun is about 2 feet of 6mm tubing, designed to shoot Airsoft BBs. I was wondering if anyone knows what the ideal amount of rubbing alcohol to use was (I am using 70% alcohol now, but i can get at least 90% at the store). I could also possibly use butane or propane, if that is better. I went on the internet and looked up the stoichiometric ratio for isopropyl alcohol, but that just told me "1mol fuel to 4.5 mol oxygen at 100 percent concentration." I am not mathematically or scientifically capable of turning that bit of info into a usable amount. 

ilpug (author) 6 years ago
I have changed the design somewhat. The gun itself will still shoot Airsoft bbs at people, which is what it is designed to do. i might shoot tic-tacs instead.
rickharris6 years ago
I imagine that you are capable of starting with a very small amount and slow;y increasing it until you reach optimum??

You need vapour to burn explosively.

It goes without saying of course that this is fraught with danger, Setting light to yourself
Bursting the tube
Hitting someone/something with a high speed BB
ETC ETC.