Call of Duty 4: Modern Warfare/Assault Rifles

The M16 is a semi-automatic Assault Rifle and is usable from the start of multiplayer. It shoots 3 bullets per shot, and is a good starter weapon for new players.

The G3 is a single shot Assault Rifle. It's bullets are very deadly, and is can cause quick deaths with well aimed shots. The G3 uses sniper rifle ammunition, so you can get ammo from fallen sniper rifles.

The M14 is the unscoped version of the single-player M21, and in its own right a very good long range rifle, though not fully automatic. If you can, go for headshots as they are one shot kills, one of the drawbacks is a relatively small magazine and the time in-between shots is a bit long. Even though you get this gun later on in the game, it is not really much use, it is fully automatic but fires quite slowly and the bullets are fairly weak, to top it off, it has no attachments to choose from.