I'm kind of with Morrus on this. Every time I read a thread about how guns aren't lethal enough, I just think:
"I'd far prefer to be shot with a typical handgun wielded by a typical trained user at (say) 30 feet of range... rather than a guy who can press 140 pounds striking a solid blow against my unprotected body with a sharpened 4-foot length of steel."
The former might kill me, but there's a solid chance it won't. And if I survive it, there's an even better chance I'll return to physical normality over the next 3 months (assuming it didn't take out a major organ or nerve system).
The latter will basically end my life. If by chance I survive the blow and associated massive blood loss, there's a very high probability that I've suffered a crippling injury or disfigurement. And I don't even want to know what it would be like to be hit by (say) an ogre using a battleaxe.
D&D ultimately abstracts all of that guff using hit points. If you want a critical system that more closely matches real life, then expect to see typical humans being killed, permanently maimed or laid up for months on end every time they take a hit. From anything.
...
However, in the spirit of being constructive, here are some thoughts on how a fantasy world would deal with it.
Firstly, not all guns can punch through all armors... and you don't need a gun to do so. A longbow arrow will punch through plate armor... where a small caliber bullet might bounce off. An ogre swinging a pick-axe doesn't get any special "armor penetration" abilities in D&D... but, if such a thing were possible in the real world, I can guarantee you it'd have penetrative abilities that would make guns look like jokes. So, if you're going to give guns special properties to ignore AC, I'd suggest you'd need to do similar for anything that could apply the same force over a small area. Manticore tail spikes, the sting of a giant scorpion... basically, any supernaturally potent piercing damage.
Secondly, I'd imagine that any fantasy world that had to deal with firearms would develop new types of armor with ablative properties. More... curves? Fewer shot traps? I'm not an armour expert, but I recall that's one of the factors that heavily influenced tank design as WW2 progressed. Tanks started developing more curved surfaces to increase the chance of deflecting shots (reduced force on flat surface). Perhaps the same concepts could be applied to plate mail. I'd certainly expect more plate and less leather/chain.
Thirdly, I'd expect wizards would be at even more of a premium than before. An area-effect anti-projectiles spell would be a huge military advantage... but, then, the same is true if you're just using arrows.
Protection from normal missiles would have been just as relevant at Agincourt as it would have been at Stalingrad. So, I guess, probably no change. It'd be nice to have magic that could protect against bullets, but it's equally nice to have magic to protect against
fireballs or ogres-with-pickaxes. It's just one more damage source that can kill a dude. If guns were so widespread that they appeared in every field battle, and if (illogically) they were the only things with special armour penetration capabilities, then I'd suggest most soldiers would simply stop wearing armour... rather than politicians spending precious gold to protect against it.
On that line, the economic considerations would be the most interesting. I think the biggest impact of firearm availability isn't going to be in terms of combat rules (...because there are so many equally lethal things in D&D), but in terms of backstory. Any critical component that goes into the firearm or its ammunition becomes a valuable resource. If black powder / copper / pixie dust is used in the production of "firearms for the masses", then control of that resource becomes important. Mining becomes key. Adventurers are hired to find, acquire, steal or destroy firearms supplies. It basically becomes a gold rush. The trivial power of guns is irrelevant to adventurers (who can throw
fireballs, heal gunshot wounds with the wave of a hand, and summon elementals who simply can't be affected by them), but their value to non-adventurers is immense. Again, much like the real world. Part of the mystique of guns is that they can turn any shmoe into a potential killer... whereas it takes a bit of training to become Robin Hood with a bow.
...which in turn suggests that the average adventuring party would actually be anti-guns, and would probably actively look for ways to destroy production or resources. Why give a 0-level peasant an even chance? If I was a highly-trained swordsman, I'd fear being made redundant by convenient mass-produced weapons. I wouldn't wait for the moment when I'm testing the strength of my plate mail or the wizard's warding spell against firearms in combat... I'd be using my powers to destroy the capability to produce such weapons before they ever got off the production line.
...which in turn suggests a campaign setting where racial or cultural violence is commonplace against any civilization which promotes the use of firearms, before they begin replacing magic or skill-based weapons. The dwarves are building cannon production facilities? Time to slap them with crippling economic sanctions, covertly assassinate or replace their leaders, sabotage their facilities or, at worst, declare war before they can produce (or proliferate) such game-changing weapons. Once the weapons exist so widely that spells and swords are no longer applicable... congratulations! You're now playing Axis & Allies, not D&D.
