Here is the thing about using guns in Call of Cthulhu: Guns should not be the answer to any and every problem that pops up. And they certainly should never, ever help you against monsters of the Mythos.
That is not to say that guns don’t have their place in the game. If a group of Cultists are trying to run you down in the alley Billy’s Bar and Grill, by all means, get out your .22 and squeeze off a few rounds. My point is they should never be the answer for the Big Bad in the story UNLESS a human has been taken over by a Mythos creature and you are using guns to destroy the human host. A well-aimed slug from a pistol may incapacitate Mr. Joe Evil, but it won’t hurt Nyarlathotep. All you’re going to do is piss it off and make it focus its attention on you. And once Nyarlathotep is aware you exist, you might as well get out your D6s and start rolling up a new character.
I’ve played with a few folks where their answer to *anything* was to pull out the guns. And I do mean guns, plural, with an s, as it was never just a single weapon with these guys. While this often helped during some battles, it often hindered us BEFORE the battle. A true example:
There were four of us playing Investigators. Due to the needs of the story, two of us remained behind in an underground cave with The Big Evil. The two others, one of who was Mr. I_Sleep_With_My_Gun, left to go get a needed item. They came across a man in a dark robe who was kneeling and praying. Mr. I_Sleep_With_My_Gun shoots the guy in the head before the other Investigator can even find out who this guy is, or before the guy can even say hello. We found out after the game the guy in the robe was the one person who knew what we were dealing with and had the answers how to help us defeat the The Big Evil, but we never even got a chance to find that out. It’s hard for a NPC to give information when their brains are splattered all over the ground.
Granted, our Keeper could have found a way to get us the information the man in the robe had, but I’m glad he didn’t. Even though it ended with some of us taking heavy damage and the rest of us with some potentially huge Sanity losses, he let us go into the final act completely blind. In my opinion, this was the right call, because, again, guns should not be the answer to every problem, nor should they be used to eliminate what one of the players thinks is a potential problem.
Call of Cthulhu has preferred ways of dealing with Mythos creatures: old tomes, dark spells, sacred rituals, etc.. You can use your guns to keep cultists at bay while the ritual is performed, or while the incantation is read, but don’t use them to shoot at gods. Such arrogance that a mere human weapon could harm an Outer God should, in my opinion, be severely punished. The beauty of Call of Cthulhu is this is built into the system. Just let the game play out as normal. Unprepared Investigators don’t last long. They will be dead or siting in the corner drooling and muttering about “those godawful frog things”.
Guns can and will be used in CoC and that is perfectly fine. They are listed in the rulebook after all. Use them in their place and use them in conjunction with the spirit of the game. They should not be the player’s crutch in any situation, and they definitely should not be the first line of offense or defense.