How are gun barrels made?

How are gun barrels made and how can they withstand the extreme pressures caused by a powder ignition? They seem to have fairly thin walls, and are composed of stainless steel (not sure which alloy), but they can hold in 30,000psi! How is this possible? I know some small caliber barrels are made from carbon fiber, which has a mive tensile strength, but how do the larger caliber barrels hold in so much pressure?