Thread: What materials can keep a high pressure of air in a water bottle?

Me and my cousin want to build some small guns that propel the bullets my air, because buying gas is too expensive (and a waste of money). And I can't think of any materials that are air tight materials that can keep the high pressure of air in the bottle. Can you please what materials can be used? (And if you can) Where to find them?

Steel bottles are the best and you can try empty SF6 gas cylinders which are usually scrapped after using the gas for circuit breakers