The Cold War saw the rise of the Assault Rifle - a weapon which combined the lightness and rapid firepower of a sub-machine gun with the range and hitting power of a traditional rifle. The original 'assault rifle' was the German MP44 which was in use at the end of World War Two. This is the weapon that gave Mikhail Kalashnikov the inspiration to produce the AK series of assault rifles.
If you look at soldiers in photos of the 1940s, they're carrying bolt-action rifles (e.g. Lee-Enfield), semi-automatic rifles with small magazines (e.g. M1 Garand), or sub-machine guns (MP40, Sten, Thompson). If you look at photos of soldiers in the 1950s, 60s, and 70s, they're carrying AKs, M16s, FN-FALs, etc.