You don't make a semi-modern firearm without semi-modern tools!
What is the tech level in these rofa(ish) series, it bothers me to no end. You have magic and swords and then suddenly some soldiers pop up with mid 19th century rifles in good chunk of them. Not to mention just casually having semi-industrial products that shouldn't exist considering the overall tech level of the world... I know we are meant to ignore it, just like one shouldn't pay attention to lacklustre or nonsensical city/castle designs, but God does it bother me.
Just take this little "invention" an automatic reloading gun. That is essentially a repeating nay an automatic pistol with infinite ammo, so why not just make a gatling gun at that point? Whether the author puts any thought (none in this case) or a lot of, so many fantasy authors when deciding to introduce magic technology just miss out on the implications of it. Either the issue of how they managed to manufacture the mundane parts of the tool or the implications of such manufacturing would pose on a world.
Now I am reminded of some semi-good webnovel I read, where the MC could use magic to conjure up elements, even heavier ones. They ended up using high-explosive bombs in some war they wound up in and it just got me thinking, is that all you can conjure? Why not just marathon conjure some U-235 and drop a gun type fission bomb on the enemy capital. That should result in peace through superior firepower you were after...
And let's not kid ourselves, this gun will just be an useful gadget in some coming high-tension assassination scene and then will never be mentioned again. No one else except MC and maybe the princeling will ever be seen wielding it. Hell, if this story continues beyond this arc, I bet it won't even pop up ever again.