LogFAQs > #977224130

as to the progression system, you can just do the zombies dailies then farm contracts. 3 contracts = 1 daily. You can grind out a bunch of contracts within the 45 min period of zombies. You can also just pick up guns and equipment in zombies and exfil with it to unlock it in the main game. I literally only have a handful of weapons left to unlock and i'm not even level 55 nor did I play MW2 much.

Just make a mental note of the guns you want/don't have, pick it up from either wall buys, loot stashes, mystery box(this is a good one because it's relatively cheap AND gives you a high rarity weapon for bonus damage) or mercenaries that you kill. Sometimes reward caches drop weapons too.