I know people keep saying pressures but it really translates to load.
Guns are designed to carry a load.
So what is the load, well it is pressure times a surface area.
Obviously you increase the pressure for the same surface area, the load goes up.
However, when you blow the case head and gas escapes into the mechanism, the surface area goes up.
Assuming the pressure stays the same (and it does not) you can imagine that doubling, tripling the surface area the gas has to push against, the load goes way, way up.
I have had bicycle tires that operate at 125 psia, does not seem like much.
But you put that 125 pounds per square inch across the side of a house, (say 60 foot by 12 foot rectangular surface) and the wall completely disappears.
Says here, that 10 psia overpressure will blow your arms off.
http://en.wikipedia.org/wiki/Overpressure
Stay away from Thermonuclear bombs.
Four (4) miles away from the epicenter the blast of 1 MT Bomb, overpressure is 6 pounds per square inch. Winds are 180 miles per hour. All standard houses and buildings "implode" with that overpressure and then are swept clean with the blast winds. (2) Pounds per square inch crush a house.
http://www.parowanprophet.com/Nuclear_War_Comes/May_25_83_Revelation.htm