[quote=Rhymer]
Gunpowder is mainly used in naval battles but some infantryman might use them, much to the distaste of traditional knights. Muskets are not exactly in use, (although a character might invent it) but a precursor, the arquebus or harquebus, is similar. Arquebus' use a matchlock firing mechanism and are not very powerful, only capable of piercing plate armor at very close quarters. Larger ships cannon are capable of devastating barrages though.
[/quote]

Arquebi, we meet again.
In fantasy, things always get a bit woozy when guns get mentioned. They had to add DLC just to put crossbows in Skyrim. In Assassin's Creed, I just shot every guard instead of swordfighting like a good Assassin. In Shamer's Daughter (a book set in the Middle Ages) everyone used swords and crossbows. Hell, even in The Saga of Darren Shan, which is set in the modern day, they had to take an oath never to use ranged weapons.
I think it should be gunpowder, not magic, we should worry about becoming OP. And, as GM rightly said, the best way to do that is to make certain armour resistant to it. Same with magic, actually. Maybe light armour resists magic and heavy armour resists bullets.

I also think certain artifacts should be more powerful than people. For example, an amulet that makes the body immune to fire should not be bested by a spell cast by a fire mage, because you could always just stab the amulet wearer.