Supergun Launching of Satellites

Other answers don't mention the fact that no single impulse (e.g, like being fired from a gun) can launch a projectile into orbit. A purely ballistic projectile fired from a gun must either crash back into the planet, or it must escape from the planet altogether.

In order to achieve orbit, at least two impulses must be applied to the projectile. The first one (from the gun) launches it into an elliptical trajetory that returns to the surface, and then the second impulse must be applied by a rocket motor to "circularize" the orbit at the moment when the projectile reaches the apogee of the initial ellipse.

Anything launched into orbit by such a gun needs to travel at orbital velocity (in fact above orbital velocity) in the lower atmosphere. That's generally undesirable, to put it mildly: there will be really serious heating.

Aside from the interior ballistic aspects of these various projects, it was quickly realized that any satellites launched by gun would have to withstand high g-loadings during firing of the gun and the size and mass of the satellite would be greatly constrained by the dimensions of the bore of the gun and the maximum impulse which could be provided by the propellant without damaging the gun.

Special designs for satellites were prepared so that sensitive electronics would not be damaged by being fired from a gun, and recognizing that the gun could not provide sufficient velocity to reach orbit, satellites with booster rockets were designed to fire after being flung aloft by the gun.

The project ended for various reasons, some budgetary, some political. The escalating war in Vietnam caused funds for a lot of research projects to be cut, and this project was originally a joint effort between the U.S. and Canada. When relations between the two countries hit a rough patch over differing policies regarding Vietnam, the project became ripe for being eliminated.