Maybe you already answered or have a post based on this, but a thought process ran through my head just a bit ago: The gun developed in a world of magic. I bring this up cause I often see folks think about magic and guns separately, or how magic itself could use guns (enchanted ammo), but I'm thinking practically across the board: How would the rise of the gun effect things like magical armor? maybe even how it would effect spell crafting and research. Just thinking beyond the gun itself...
In order to properly answer that question, we’re going to have to step back and look at why the gun rose to dominance in the real world.
The popular narrative is that guns came to dominate the battlefield because even a primitive firearm can tear through heavy armour like tissue paper, rendering armour militarily irrelevant and signalling the end of the era of heavy cavalry, but that’s not actually true. In the period when plate armour was popular, it was totally possible to make a suit of armour that could stop a bullet from the firearms of the day. Indeed, it became common practice to “prove” a breastplate by discharging a pistol at it at point-blank range, with the resulting dent serving as evidence that it had passed the test.
(This is where we get the term “bulletproof”, incidentally.)
So if that’s the case, then why did guns ultimately result in the disappearance of heavy armour from the battlefield?
Believe it or not, the answer is “economics”.
Straight up, heavy armour is expensive as hell, and fighting effectively while wearing it requires long and arduous training. Conversely, you can train someone up to be a halfway-competent musketeer in just a few weeks, and guns could be had comparatively cheaply. Yes, cheap firearms were extremely dangerous early on, but with the composition of armies trending away from small groups of elite warriors and toward massive regiments of minimally trained conscripts, if some poor musketeer’s gun blows up in his hands, you just replace him with the next guy down the line.
When you can field twenty expendable riflemen for the same cost as one armoured knight, and the armoured knight certainly isn’t twenty times more effective than the rifleman, well, it’s easy to see where the smart money lies - but in a fantasy setting, any or all of the assumptions that get us to this point could change.
In short, in order to answer the question of how firearms would integrate into a D&D-like fantasy setting, you first have to step back and answer the much broader question of how the existence of D&D-like magic would influence the economics of war. Good luck!
(It also pays to bear in mind that the answer to that question might be different for player characters than it is for society at large. Costs that are unbearable when equipping massive armies could be pocket change to a successful adventurer, and dungeon crawls typically don’t take place in the sorts of environments in which early firearms can best bring their strengths to bear, so it may well make sense for your paladin to be running around in bulletproof plate even if it doesn’t make sense for the Queen’s army.)
