First time GM about to run a game of Call of Cthulhu. Can someone please explain to me how guns are balanced against each other in this game?

I understand that there's a large variety for realism and flavor, but there's so many differences between them mechanically and from what I can tell there's little to no downside to just taking the strongest gun every time. What I'm asking is, if the players are going to be carrying a single-shot rifle, what reason is there to pick anything other than an elephant gun, for example? Alternatively, if they're going to carry a shotgun, what's the point of ever using 20-gauge rather than 12-gauge?

What am I missing here?