Yo Kass answered
Because World War II ended less than 5 years before the outbreak of the Korean war, the weaponry used during the Korean conflict consisted primarily of leftover stock from World War II. This was supplied by the 'sponsor states' of the two Koreas: The United States and the Soviet Union.
Weapons of North and South Korea
The Soviet-backed North Korean forces used Soviet-made weapons, and had an arguable advantage over the UN forces in several ways. Most notably, the standard of arms carried by communist North Korean rifle companies was superior to the small arms weaponry of their South Korean enemies.
This was mainly due to the U.S completely halting development and production of small arms in the post-WWII period - due to the American administration's view that the nuclear age had made ground warfare a thing of the past.
Why the Americans were unprepared for war
The decision to halt production of weapons for ground warfare was made chiefly by American president Harry Truman and his secretaries of defense James Forrestal and Louis Johnson.
They also instigated changes in army training methods and effectively disbanded the Marine divisions of World War II in the belief that nuclear armament was the way forward.
This meant that the Americans were only able to provide a skeleton force of the Marine Corps - wielding out-of-date weapons - to face the North Korean and Chinese forces, and ultimately resulted in the unnecessary loss of many lives.
The guns & weapons used in the Korean war
The list of weaponry used in the Korean war is extensive. Although both sides relied on surplus World War II stock, the effectiveness of their arsenals is reflected in the tragic death-toll of 3 million people who lost their lives during the conflict.
My personal favorite, as far as Korean war weapons go, is the trusty U.S. Issued M1 Garand rifle.
For a comprehensive list of the weaponry used in the Korean war, check out the following links: