Question: how much xp does it take to level up in modern warfare?
Answer: Yes, you read that correctly. Every firearm in Modern Warfare can be leveled up to level 70 by accruing weapon XP through use. Every time you level up a weapon, you'll unlock either an attachment for one of that weapon's Gunsmith categories (Muzzle, Underbarrel, Optic, etc.) or a weapon perk.