Well, in my PnP campaign I do have firearms in it. From spyglass rifles, dragon pistols, and hellsbreath rifles, and the such. It all works well in DnD format. Ptolus is a damn good setting. I would love to play a dual pistol shooting wizard, mixing in both technology and magic. That is one of the main drawbacks of standard DnD. It just doesn't take in account that technology, even if powered by magic, advances.
Hellsbreath rifles are pretty cool. It is basically a flame thrower that uses alchemist's fire as the ammunition.