[Flieger]

It the payload is a solid lump of depleted uranium...

The issue is that all the energy needs to be imparted to the projectile while in the barrel. This means huge g forces that would destroy any payload like a satellite. Rockets still pull some good g forces but they are tolerable since they have more time to reach orbital velocity which means the impulse is the same but the peak force is less.

Multi-stage rockets are sort of like using a gun to give the payload a little help. Part of the propulsion module does not make it to orbit, just like how the gun stays on the ground.

From the Earth to the Moon used a gun built into the Florida ground to launch the astronauts but Verne describes how violent the launch was, even with the many levels of water and collapsable wood floors that were used to cushion the astronauts on launch.