It is said that Gunpowder was developed in Harchong a little less than a century before the events of OAR, but when Nimue awakes ships are outfitted with cannons as their standard weapon and soldiers are armed with matchlock arquebuses and rifles. On earth the first instances of Gunpowder being used happened in Tang Dynasty china about 850 CE. Over the next century during period of civil war that lasted for 60 years, this was used in the first firearms which were to the effect of this...

...simple one shot bamboo tubes loaded with gunpowder and set off with a match. It took another 400 or so years of experimentation through intermediate steps like this...

...until you got something like this.

This design evolution was done by trial and error by people who did not have inquisitors with the proscriptions of Jwo-Jeng breathing down their neck.
My point is that guns seemed to evolve remarkably fast on Safehold even before Merlin made his appearance.
Zor