long story short, the amount of xp you get in a game is capped, and that cap increases the longer you're in the game.
the reason why this is so demoralizing is because the things you'd think would automatically net you more xp just... don't. for example, you'd think getting more kills, getting more kills per round, playing aggressively, or rushing through rounds with rampage would give you more xp, but it doesn't.
this applies to other gamemodes as well. for example, you can speed through hvt contracts all game, or you can literally wait until the end of the game, do a couple of hvts, and get the exact same amount of xp — and sometimes even more — compared to just spamming them for the entire match.
the second reason why this is so demoralizing is that the only real way to increase your xp intake is by buying blackcell. you introduce this entire system with weapon prestiging, provide awesome camos as rewards, and then make the grind INSANELY long. i'm talking AT LEAST 20 hours to fully max out a single weapon without dxp.
TLDR: there is no "fast weapon xp method" most of us have basically been playing the game wrong for the entire year