Yes, it certainly would be; it just hasn't been done yet. The concept of launching a satellite into space using a cannon was conceived of as far back as 1678 by Isaac Newton, in his Principia Mathematica, where he used the concept of an orbital cannon as a thought experiment to illustrate the principles of gravity. In 1865, Jules Verne, the father of science fiction, wrote a story, From the Earth to the Moon, in which a group of daring men used an immense cannon to fire themselves to the Moon.
It wasn’t until World War I that so-called “superguns” actually started to be built. The Germans constructed a “Paris gun,” a 28 meter-long gun capable of firing a 105-kilogram shell over 120 kilometers (75 miles). The idea was to hit the city of Paris from the safety of the German border, and the project was successful. Even though the payload was relatively minuscule and the gun was inaccurate, its main effect was psychological. This was the first time such a colossal gun had been built.
The Germans built superguns for WWII as well, such as the infamous railroad gun, Big Bertha, which was again used to pound Paris into submission. However, big guns turned out to be relatively impractical for warfare, as they demanded a large military detachment for protection and were either stationary or slow-moving, making them easy to locate and destroy via air strikes.
From the 1950s until his assassination in 1990, the field of superguns was dominated by one rogue physicist turned arms dealer – Gerard Bull. In 1961, Bull built a 36 m cannon with the help of the US Navy, and throughout the 60s, used it to launch over 200 atmospheric probes up to altitudes of 180 km (112 mi). This established the feasibility of using cannons to launch payloads into suborbital trajectories.
Much later, in 1988, Bull was contracted by the Iraqi government to build a “Project Babylon” supergun: an extremely long-range artillery piece. Although Bull was assassinated before the gun could be completed, a “Baby Babylon” gun was built with a 45 m barrel and a range of 750 km (466 mi). The complete Project Babylon guns were to be 156 m long, and, if they worked, would have been capable of launching a 200 kg payload on a rocket-assisted shell into orbit at a cost of just $600 US Dollars (USD) per kg. But the components of the incomplete Babylon Gun were subsequently confiscated or destroyed by the United Nations during the 1991 Persian Gulf War.
More recently, in 2007, a visionary aerospace engineering graduate from MIT, Ben Joseph, founded a company called, Ballistic Flight Group LLC, to build the first cannon capable of launching a projectile into orbit. The goal is to use a combination of ramjet and conventional cannon technology to launch a 2000 kg (4400 lb) payload into orbit. This is called ram accelerator technology. If it comes to pass, this orbital cannon could cut launch costs by a factor of 10, lowering the current cost of about $5,000 USD per kilogram of payload to about $500 per kilogram. Because the forces on the payload exceed 2000 gravities, this would not be a viable way for humans to get into space, but it could be very useful for launching supplies for space stations. The cost of the cannon is estimated at $157 million USD, quite cheap by the standards of current launch technologies. It seems quite certain that an orbital cannon will eventually be built – it’s more a question of “when” than “if.”