Keeping this in consideration, what does XP do in modern warfare?
Regular XP (simply “XP”): This is how you level up your account to unlock new weapons for your Loadouts or Loadout Drops and other items that can be used within Warzone and the full version of Call of Duty: Modern Warfare.
Likewise, does plunder give XP? Plunder works similarly, but there's also a cash collected bonus, which rewards a very aggressive style of play. Don't forget that you can also gain huge sums of XP from completing Warzone missions from the main menu.
Also question is, what gets you the most XP in Modern Warfare?
Unless you're really confident in your ability to rack up kills, objective-based modes like Domination and Ground War are your best bet for consistently high XP totals. You get a decent amount of points from capturing objectives, and you're guaranteed to capture at least a few objectives in any well-balanced game.
Does Spec Ops give weapon XP?
You can earn weapon XP in Campaign, and you can even earn Multiplayer XP in the Co-Op Spec Ops mode — though not a lot of people are playing that right now.