Kalashnikovs in Korea

The AK-47 Kalashnikov rifle first appeared in prototype form in 1947 and entered production in 1949.

What if the Soviets were a bit more efficient and had enough weapons stockpiled by 1951 to share them with the North Koreans and Chinese during the Korean War.

How would large numbers of assault rifles being used against the UN Forces affect the war and military development in the West thereafter?

IMO no. The thing behind sucess of AK-47 isn't that's it's better at killing people but simple to use and easy to maintain.

If they are used US (and other UNers) would capture it and try to develop something similar).

The US had inferior tanks, inferior antitank weapons, inferior fighter aircraft, and inferior intelligence in Korea. The US had superior transportation in the form of trucks and aircraft.
If the North Koreans had had more trucks, they would have won. The US troops would not have been able to retreat fast enough to stay ahead of the North Korean army. Kalashnikovs were not as important. They might have killed a few thousand more US troops.

True it's highly unlikely that the outcome of the war would be changed but what about the way it was fought?

The land war was essencially fought with WW2 weapons. How would governments react if the troops on the ground suddenly found themselves in situations where the North Koreans and Chinese outgunned them?