[toomanybeersies]

The plan wasn't to get into orbit with the gun alone. They had designed shells that had rocket motors in them, to get them to orbital velocity. The gun was simply the first stage.

The critical problem with using a gun though, is the sheer amount of force applied when firing the gun. They couldn't even get dummy payloads to work correctly, the rockets motors warped due to the forces involved.

[castle-bravo]

IIRC one solution is to use pressurized hydrogen gas pumped through a heat exchanger instead of explosive charges. That way, the pressures and temperatures involved can be controlled precisely.