Finding the best Assault Rifle in Modern Warfare isn't difficult. There is a clear best, but you need to unlock the attachments that make it special. The gun is available at Level 6, which shouldn't take long for Call of Duty savants.
But there are other options for Assault Rifle users. They just each have a time and place.
Best Assault Rifles in Modern Warfare
Unlocked early on, you can stick with the rifle for the rest of the game as you slowly unlock better attachments and perks. Use the Operator Reflex Sight and you can smoke people at medium to long range. You may lose out when it comes to short range, but that can't be helped.
Despite the strange sideways recoil, the damage output and solid ADS time, you can win most gunfights. It's harder to master and takes time to adjust, but that's worth the risk. Don't be surprised if you suddenly start winning fights you normally lose.
The M13 is the definition of spray-and-pray. With a high rate of fire and large clip size, it functions as a SMG more than an Assault Rifle. It has better handling and range than a SMG, but much worse than the Ram-7 or M4A1. If you handle the spray pattern and learn which fights are beyond your reach, this weapon is deadly in the right hands.
Whatever weapon you choose, make sure to test it out this weekend during the Double XP and Double Weapon XP event.