space gun

The large accelerations experienced by a ballistic projectile would likely mean that a space gun would be incapable of safely launching humans or delicate instruments, rather being restricted to freight or ruggedized satellites.

Atmospheric drag also makes it more difficult to control the trajectory of any projectile launched, subjects the projectile to extremely high forces, and causes severe energy losses that may not be easily overcome. A space gun with a "gun barrel" reaching above the lower troposphere, where the atmosphere is most densely packed, may mitigate the issue.

A space gun, by itself, is generally not capable of placing objects into stable orbit around the planet.

If acceptable solutions to these fundamental issues could be achieved, a space gun could offer access to space at an unprecedented low cost.
[edit] Getting to orbit

A space gun, by itself, is not capable of placing objects into stable orbit. The laws of gravitation make it impossible to reach a stable orbit without an active payload which performs orbital correction burns to change the shape of its orbit after launch. The orbit is a parabolic orbit, a hyperbolic orbit, or part of an elliptic orbit which ends at the planet's surface at the point of launch or another point. This means that an uncorrected ballistic payload will always strike the planet within its first orbit unless the velocity was so high as to reach or exceed escape velocity.

Isaac Newton avoided this objection in his thought experiment by positing an impossibly tall mountain from which his cannon was fired. The projectile, however, would still tend to circle the planet and strike the point of launch.

As a result, all payloads intended to reach a closed orbit would have to perform some sort of course correction to create another orbit that does not intersect the planet's surface. The amount of fuel carried would thus reduce the payload-to-fuel ratio, decreasing the efficiency and increasing the complexity of such a system. It is conceivable that in a multi-body gravitational system, like the earth-moon system, that a trajectory could be found that does not re-intersect the earth's surface, although these paths would likely not be very simple nor desirable, and would require much more energy.