Competitive Call of Duty is not one game mode, it is three played back to back. A GB is a Hardpoint, then a Search, then a Control, and calling any one of them "the match" misdescribes what happened — teams routinely drop a Hardpoint and take the series, and everybody involved understands the series is the thing that counts.
That has a direct consequence for a ladder, and it is the decision this page exists to get right: the rating should move once per series, on the series result. Rating each map separately triples the movement, credits a mode specialist for a series they lost, and produces a leaderboard that disagrees with what the room saw.
The second thing that separates Call of Duty from most games here is that the competitive unit is genuinely a team. GB communities are built out of named four-man rosters that have existed longer than any individual ladder — so unlike Valorant 10 mans or CS2 pugs, rating the roster rather than the player is frequently the correct answer rather than the exotic one.
What these communities almost always lack is persistence. Results live in a channel, scroll away, and the question "who is actually the best team here" gets settled by whoever argues hardest. That is the gap a ladder fills, and it does not require changing how anyone already plays.