I just read the FRA build rules. They limit gas bottles to 50 psi without certification. You can always test these things. The biggest problem I had with coke bottles was the lids blowing off.

Quote from Giant Robo

I use epoxy in El Bravos` bottle caps.
Make the pipes going into the bottle about an inch or two long so the ends don`t get glue in them.
Sand the inside of the bottle neck so the glue will stick then completely fill the cap with epoxy and screw on the bottle from above. Hold the bottle upside down until the epoxy cures. You`ll feel the cap get nice and hot.
Your`e pipes will not leak or blow off and it strengthens the cap.
Make sure you mix, pour and screw on the bottle quickly before the epoxy starts to go off so it is still runny and will flow and seal the neck.

For El bravo I use two 1.25 litre PET bottles. They were fine at 100psi but to go higher with confidence I covered them in fiberglass and also the caps were filled with epoxy before being screwed on so they don`t leak and won`t blow off. I plan to give them another layer of glass; three all up and within the safety of the arena, pump them up to 200psi and then have another bot bash them around a bit. Then, if there`s no big bang! I`ll feel safe to put them up to 150 psi regularly.