GnomeWorks
Adventurer
I think part of the reason why it's hard to accept in D&D is less the fantasy setting and more hit points.
HP is kind of ridiculous whether guns are involved or not.
I can see why people might assume that guns are more dangerous than swords. I also think those people both overestimate the deadliness of guns -- there are plenty of real-world stories of people getting shot in ways that aren't life-threatening, or that keep standing even when shot several times -- and underestimate how dangerous a three-foot-long length of sharpened steel is.