Artifact refining, like cultivation, is exceedingly unscientific, yet profoundly idealistic.
It's like the Orc Alchemist from the past who could create a tool in accordance with his thoughts simply by declaring, "I have an idea."
Artifact refining functions similarly, Magic Artifacts need a slightly more concrete design blueprint, but when it comes to Spiritual Artifacts, no matter what the appearance turns out to be after refining, they will ultimately possess the functions the refiner desired.
Therefore, even if Dong Tianyu did not have a detailed blueprint for a rapid-fire cannon, as long as he understood the principles during the refining process, and had a general idea of its internal structure, along with adding the appropriate materials, as long as the refining was successful, he would obtain the Magic Artifact he wished for.