Shrapnel isn't really an issue when PET bottles explode, they tend to tear as opposed to shattering. The best protection you need in that situation is a good set of earplugs
Tru, you're right. although if you have it leaning up against your body and the bottle is right up to your side, things could go wrong. (yeah, there's relatively nothing to worry about when it comes to FLYING shrpanel).
My first potato launcher, I was in sixth grade and doing a comparison of combustion verses pneumatic cannons, the website I had gotten plans off of, the guy posted pics of what had happened to his chest because of a soda bottle reservoir. It was bruised and had a pretty nasty cut.
So that's just what I was thinking of when I said shrapnel.