I generally feel the Blast Cannon is too expensive and too bulky for the limited number of shots it has by default. The Blast Cannon costs $5500 loaded in its basic form, takes four spaces, hits on a base of 7 and has 10 shots. You can make the ammo HESH for $500 more, but HEAT ammo is prohibited as being too powerful.
You can add an extra magazine to boost the number of shots to 20, but at five total spaces, you can only put that in a non-sloped non-streamlined sedan or larger vehicle. That is not much of a problem itself. It just limits your options. If you want to add Component Armor to that facing, your up to an non-sloped non-streamlined luxury as the minimum.
The Blast Cannon is a fine weapon and definitely has its place in considering offensive options. I have been surprised by people's near obsession with the weapon - to the point where people seemed to believe any other weapon option was substandard. When I went to GenCon for the 2050 Tournament, I was surprised by the number of people using the Blast Cannon in the Preliminaries. In the Finals, four of the eight duellists had a Blast Cannon, either Front or Side mounted. (Two had triple linked Rocket Launchers. One was Laser Guided. I believe that was a holdover from the mid to late 2030's when LGL and Rockets were the rage - Expanding on that is for another post.) The Blast Cannon, with or without HESH, is great for punching through thick metal armor, which also seemed to be very much in vogue at the time. I had a sloped sedan with a turreted VMG with HD ammo and a FCGS with an extra magazine. I kept my speed up to maximize targeting penalties for the big guns and tried to keep my distance until I could close in and use the Flameclouds. I never took a single hit from the Blast Cannons, only from the Rocket Launchers. (I also finished all of my kills in that duel with the FCGS - Writing about my love of Flameclouds is also for a future post.)
2051 was similar. Again, in the Finals, four of the eight had a Blast Cannon. Again, I had a turreted VMG and FCGSs. Again, I never took a hit from the Blast Cannons.
The Blast Cannon is accurate and can readily punch through armor. It is a good option to try to get to a quick kill count in AADA convention scoring. It is expensive and bulky and not suitable for long fighting with only ten rounds. The Blast Cannon is a fine weapon. I just don't feel it is the pinnacle of Car Wars offensive technology.