While I don't disagree with the OP regarding how underpowered guns seem, isn't the whole point of Call of Cthulhu to encourage the players to flee for their lives rather than to stand and fight?
I doubt that it is the whole point. The whole point involves creating an experience of fear and horror, one result of which might be having the player characters flee for their lives rather than standing and fighting.
But what I'm noticing with 20 more years experience as a game master is that the assumption that characters will flee for their lives as a rational choice is based on several assumptions, one of which is that the players are playing along with the device and are woefully and perhaps deliberately underequipped with light firearms and some improvised melee weapons. Such equipment would leave them not just woefully underequipped to deal with the supernatural horrors of the mythos, but ordinary animals such as grizzly bears, moose, cape buffalo, rhino, elephants, lions, and tigers.
If on the other hand, the party is equipped with state of the art hunting weapons and the accoutrements of warfare in the 1920s, then not only are they much better equipped to deal with large game animals, they are equally equipped to deal with the majority of monsters in the mythos that characters are thrown against: byakhee, hunting horrors, shantaks, zombies, deep ones, and so forth all are no harder and often much easier to kill than published stats for cape buffalo, rhino, and elephants. If firearms have more realistic damage to large creatures, so that a guy with a large game rifle really can realistically hunt large game, that gap gets even smaller. Half-damage or moderate armor is really only a problem when the party is using small caliber personal defense weapons. Minimum damage from impaling attacks is really only a problem if the party doesn't have semi-automatic and automatic weapons.
This raises an issue for me that is not usually addressed. All that research ability is supposedly there so that the party won't get into combat. But the party needs to not get into combat primarily because it's assumed most of its points are in research skills rather than combat skills and that it's woefully underequipped. That's not going to be a reliable assumption in the long run.
Now all that being said, it would still be trivially easy to kill CoC PC's. Most CoC monsters have horrifying attack modes once they get into tentacle range, and CoC investigators never have a surplus of hit points. Plus, if investigators have a huge asymmetrical advantage against most monsters in terms of ranged firepower, then monsters generally have a huge asymmetrical advantage in terms of magic with numerous low cost attack spells that can really mess a party up. It's easy to design scenarios where monsters can launch attacks from concealed positions and at close range, or where monsters have actual immunity to firearms and not just resistance to them.
But, I'm still looking for keepers with experience with players gunning up with optimized weaponry and high weapon skills - bayonet tipped trench shotguns, Thompson submachine guns, African big game rifles, and so forth. My suspicion us you can just roll with it until overconfidence gets them in over their heads, and that for many high combat scenarios that upgunning is actually essential and desirable. So many keepers seem to take it for granted that you can't outgun mythos monsters. As far as I can tell, not only can you, but being able kill mythos monsters before they are able to act may be one of the most rational strategies players could pursue. There is no way for example to defeat a Dark Young in close combat as its attacks are just too powerful. But with enough firepower and a round or two to kite the thing before you get into tentacle range, it's just a sanity draining elephant and goes down against big guns the same way.
