Some Creatures you might want to fight with guns
Holy, Unholy, weapon needed for some = count as magic
Cold-Iron for some.
Angel, Astral Deva = Touch AC 14 = DR 10/evil
Angel, Planetar = Touch AC 13 = DR 10/evil
Archon, Hound = Touch AC 10 = DR 10/evil
Archon, Lantern = Touch AC 11 = DR 10/evil
Archon, Trumpet = Touch AC 13 = DR 10/evil
Bebilith = Touch AC 9 = DR 10/good
Behir = Touch AC 9 = DR none
Bulette = Touch AC 10 = DR none
Chimera = Touch AC 10 = DR none
Chuul = Touch AC 12 = DR none
Demon, Babau = Touch AC 11 = DR 10/cold iron or good
Demon, Glabrezu = Touch AC 8 = DR 10/good
Demon, Hezrou = Touch AC 9 = DR 10/good
Demon, Nalfeshnee = Touch AC 13 = DR 10/good
Devourer = Touch AC 12 = DR none
Dinosaur, Ankylosaurus = Touch AC 8 = DR none
Dinosaur, Brachiosaurus = Touch AC 6 = DR none
Dinosaur, Elasmosaurus = Touch AC 11 = DR none
Dinosaur, Stegosaurus = Touch AC 10 = DR none
Dinosaur, Triceratops = Touch AC 7 = DR none
Dinosaur, Tryannosaurus = Touch AC 7 = DR none
(white, brass, copper = same as black)
Dragon = Young black Dragon = Touch AC 12 = DR none
Dragon = Adult black Dragon = Touch AC 10 = DR 5/magic
Dragon = Ancient black Dragon = Touch AC 8 = DR 15/magic
(green, red, bronze, gold, silver = same as blue)
Dragon = Young blue Dragon = Touch AC 10 = DR none
Dragon = Adult blue Dragon = Touch AC 8 = DR 5/magic
Dragon = Ancient blue Dragon = Touch AC 5 = DR 15/magic
Dragon Turtle = Touch AC 8 = DR none
Earth Elemental = Touch AC 10-7 by size = DR none to 10/- by size
Giants (verys) = Touch AC 8-11 by type = DR none (all)
skip, skip, skip,
Kraken = Touch 6 = DR none
Linnorm, Craf = Touch 10 = DR 10/cold-iron
Linnorm, Ice = Touch 8 = DR 15/cold-iron
Linnorm, Tarn = Touch 10 = DR 20/cold-iron
Neotheld = Touch 4 = DR 10/cold-iron
Sea Serpent = Touch 8 = DR none
Shambling Mound = Touch 9 = DR none
Slug, Giant = Touch 4 = DR 10/slashing or piercing. (bullet can do blunt)
Treant = Touch 7 = DR 10/slashing
Xorn = Touch 10 = DR 5/bludgeoning (bullet can do piercing)
.................
Just a short list, left out a lot.