An Assault Rifle is a classification of weaponry. Assault Rifles are usually automatic weapons but can be single-fire, but not as frequently seen. Assault Rifles started being put into service in World War II. The first "true" Assault Rifle is the STG-44, used by Germany in World War II.
Assault Rifles are influenced by Submachine Guns, with their fully automatic fire. The first assault rifles were the Maschinenkarabiner, Sturmgewehr & AK-47 models. The Maschinenkarabiner and Sturmgewehr models were both introduced by Germany. The Maschinenkarabiner was never put into service, however, but the Sturmgewehr (STG) was used by Nazi Germany during World War II. The AK-47, however, was designed by Russian Mikhail Kalashnikov and had inspiration from the STG, but was quite different. Some think that Red Army units were fielded with the AK-47 at the end of World War 2, but, as the name suggests, it was first made in 1947, two years after the fall of Nazi Germany.
After seeing how useful fully-automatic weaponry was, the USMC and US Army began conducting experiments with Assault Rifles. They introduced the 7.62x51mm NATO cartridge (almost the same as the .308 Winchester, but with very minor differences.) The weapon designed for these cartridges, the M14 was capable of automatic fire and semi-automatic fire. But the high recoil of the weapon made it almost impossible to use in full-auto. The 5.56x45mm NATO cartridge was then made to make it easier to fire full-auto. As the US forces started to introduce the M16 they saw how it had a very high tendency to jam, so they put the M14 back into service while they started to fix the weapon. The M16 was then used in the Vietnam War with great success.
Over the next 20 years, assault rifles would be expanded on, and would eventually be able to have things such as grenade launchers, optics, and suppressors attached to the weapons. Assault rifles are used in modern military units and are standard-issue in most countries.
Assault rifles are the all round weapon. If you don't know what to have go with an assault rifle. It has the damage for long and medium and short range. It does fine in all of these. There high use of attachments are also high. If you go into battle and don't know what to use, pick an assault rifle. A shotgun is only close range. A sniper is only long range. Enough said.
List of Assault Rifles in Call of Duty
*Although classified as assault rifles in-game, these guns are technically battle rifles due to their larger caliber cartridge.