The purpose of horror roleplaying is to have fun.
This sentence appears on p.10 of Call of Cthulhu 5th Edition, under the heading "Purpose of Play." It is the most important rule in the book.
"Fun" for your group can be defined many ways. If it's shotguns vs. Cthulhu, great. If it's roleplaying the tortured emotions and madness of artists and poets reading blasphemous tomes with nary a weapon in sight, also great. Both methods of play are valid. Both are possible with the same ruleset.
If the difference in hit damage between an M1 rifle and an elephant gun is important to a player, run with it. The unwritten rule is that all of these rules are optional, and use them as your group wishes. Whatever your group defines as fun is the object of the game.
The strength of a traditional RPG such as Call of Cthulhu is the ability to play anywhere on the continuum between the Most Important Rule and complete Rules As Written. It is not a bug. It is a feature.