@Robbini Cannons have always had that general long tubular shape. Most of the technological advancements in that field have been improvements on firing mechanisms, how the cannon is mounted and transported, ammunition, and the overall metallurgy. It wouldn't be until percussion caps and advancements in breech-loading that gunpowder artillery would make the leap to modern artillery styles.
At this point in time that "bell" is excess iron used to compensate for inferior metallurgy so it doesn't just explode upon ignition.