This... actually inspires very little confidence. The assassination of the archduke involved several assassins who each failed iteratively for ridiculous reasons on the motorcade route. Princip himself had decided to give up on the assassination, only to find out the cafe he had gone to ended up being directly on the motorcade path. The serendipity of his proximity was probably the only reason Ferdinand ended up dead that day.

The cannon doesn't have to work all the time, just once effectively, and possibly even accidentally.