Crossbow is a brand new weapon in Call of Duty: Modern Warfare and it is pretty powerful so you should get one. Here is a short guide on how to unlock it.
Crossbow is a one-kill weapon that can take down even scorestreaks in certain cases. It has been introduced yesterday, in the latest Call of Duty: Modern Warfare 1.13 patch and there are two ways you can unlock the weapon.
The first way is to simply purchase the weapon and unlock it instantly. It will cost you $12, which is quite pricey for a single item but it is your hard-earned money and if you do not mind spending such a sum on an in-game item, then go ahead, get yourself a shiny new tool.
...or grind for it
For those of you who do not want to spend extra money on Call of Duty: Modern Warfare there is another way of obtaining the Crossbow. It requires a lot of grinding but you probably expected that. To earn the weapon for free, you need to complete a certain challenge which requires 125 kills, 5 in 25 matches.
Just equip a Marksman Rifle of your choice, put a Reflex Sight on it and start shooting. You need at least 5 kills in each of 25 matches, which does not sound too hard but it can be if you are not a fan of Marksman Rifles. Do not ask how we know this.
There is another thing you should know before you start with the challenge. Marksman Rifle kills will not count if you join an on-going match so make sure to join a match from the beginning to make sure your kills are not completely worthless.