Of course, guns actually did increase the lethality of combat and render armor all but useless. They also massively changed the way tactics worked. Which is fine if that's what you want to do with D&D. But if they're a common weapon, expect some significant changes. If you're in a situation where the opponent has guns and you don't (like we were, because we hadn't been able to buy any), you're not going to do well.
Personally, I don't like guns in your standard dungeon crawl. How often do you have cover available anyway? You don't have a choice about going around that corner where there might be someone waiting.