Gunpowder came many times into Europe. Granted, the Mongol reintroduction probably reminded people who mostly forgot about it. And "guns" were made many times. Drawings of cannons and hand cannons came from the 11th century, which means someone must have envisioned them. And most were more likely to blow up in the user's face than actually send a projectile at what it was aimed at. It's 1500 and let's try to make a weapon out of this explosive powder despite 500 years of failure.