The project was based on a flight range in Barbados, from which shells were fired eastward toward the Atlantic. Using a 20 meter, 40cm caliber navy gun (later extended to 40 meters), the team was able to fire a 180kg slug at 3600 m/s, reaching an altitude of 180 kilometers. While the speed wasn't nearly enough to reach orbit, it was a major achievement given a budget dwarfed by most ballistic missile programs.
The program was cancelled shortly after this. Most of the criticism was focused upon Bull and whether a ballisticly fired payload could ever reach orbit, although the politics of the Vietnam war and soured Canadian/US relations played their role as well.
While it is questionable whether projectiles could ever be directly fired as a single stage from Earth without the use of exotic materials in construction, HARP demostrated an efficient way to launch a projectile part of the way. Additionally, it showed that electronics could survive such a launch, making it possible that a sabot-fired rocket could launch once it reaches its peak altitude, and continue the rest of the way into orbit.
A second incarnation of the HARP project, also conducted by Gerald Bull, was done in Iraq under the patronage of Saddam Hussein. The assassination of Bull and the 1991 Gulf War ended the project partway through development.