In addition to being able to getting to level 55 with your character in Call of Duty: Modern Warfare, you have the ability to level up each weapon in the multiplayer mode. When you level up a new weapon, you unlock new cameos for it, so it's a nice way to show off your dedication to a specific weapon. A lot of people are wondering what the max level for a weapon in Call of Duty: Modern Warfare is, and the answer is level 100.
After getting your weapon to level 70, you will have unlocked all there is to the weapon. However, there are another 30 levels to gain just to show off, so you can keep going until you hit weapon level 100. It takes a long time to level it up to level 100 though, so be prepared to get a lot of kills with the weapon you want to get to max level.