America, and other UN nations, used WWII leftovers I know. What did the North Korean and Chi-Com forces use?
I know the SKS was created in 1945, and the
AK-47 a couple years later. Were these used in the Korean Conflict?

They used WWII leftovers too, primarily provided by the Russians, or sometimes from their own stocks.  

Soviet stuff generally meant Moisin-nagants, PPSh, the other Soviet WWII SMG that I can't remember right now, DP LMG and heavier belt fed things at the company levels.  They also used Moisin-nagant Sniper rifles, and even Anti-tank rifles as long range sniper weapons to some effect.  

As for their own WWII leftovers, this runs the gamut from Mauser 98s (and 88s!), Japanese weapons, and even American weapons that we gave to the Chinese Nationalists in WWII and after.  

Several weeks ago, a good history of the Korean War was on one of the cable channels. They showed a number of PRC troops armed with Garands.

And I've seen photos of PRK or PRC "crispy critters" who had Garands laying beside them.

But Russian WW2 era smallarms were no doubt their primary arms.

According to John Toland, "In Mortal Combat", and Jim Wilson, "Retreat, Hell", the ChiComs also had a large number of Thompsons, at least in the Chosin campaign.  Wilson notes that in the cold of Chosin a lot of the Garands would freeze, not so the Thompsons.  He cites a number of instances where Marines dropped their Garands and took Thompsons from dead Chinese soldiers because of their reliability.  Neither author discusses where the Thompsons came from or how so many came to be in Chinese hands.

I remember reading where the Nationalist Chinese went nuts over the Tommy Guns even making their own Mauser Broomhandles chambered in .45 ACP for their Thompson armed units.

It must have been wierd for the GI's coming under Garand and Thompson fire. At least re-supply was easy.

According to my father who witnessed the war through a ringside seat courtesy of the 45th Infantry Div. The Red Chinese had just about every military small arm under the sun including Japanese, German, American, British, Italian, French and Russian. They were given a shit load of small arms during and after WWII. G.I.'s were likely to see the reds with everything from an M1911 to an STG-44 and everything in between. Not to mention rakes, hoes, scythes and other dangerous farm implements
That the troops bringing up the rear would carry until they could get a firearm from a dead comrade.

I don't really recall where I came up with this, but I recall that China was one of the worlds big surplus small arms importers for the century before 1949.

As I recall, alot of US civil war surplus went over there as well.

It is really bizarre looking at old photos of Chinese soldiers with Mauser rifles in Germanic uniforms surrounding Pzkw 1 tanks.  Manchurian  troops?

Roy Dunlap wrote of German munition ships making the run during the war to China that the British knew about, but did not stop as the shipment would be used to fight the Japanese.  Some ally!  
 
There was a good article in Rifle or Handloader a few years ago about the surplus Mauser 98's that came into the country a few years ago.  These rifles had been used by combat troops for about 20 years and looked the part.  

The author stated the FN rifles were more durable than the equivalent Mauser - heat treatment of small parts, don't remember.

An interesting modification of the rifles was to hammer the firing pin tip longer and sharper to reliably fire the rifle in the cold Korean winter.  The modification pierced primers every time, and one of the bolts in a picture had an eroded firing pin hole nearly the size of the primer!!!  Some poor slob had to be the last one issued some of the dogmeat I've seen (and bought).    

 

Go forth and Find the September issue Of Small Arms Review.  
Quoth the cover "Small Arms of The Korean War"

From the Article I just read, "cartridges of the Korean War"  Teh 7.62x54 was the machine gun round, 7.7mm Japanese was used in many chinese rifles, and the 7.62x39 was used in many SKS rifles.

So I suspose, Maxim machine guns, Akisara (sp?) and Mosin type rifles along with SKS's along with all the previously mentioned weapons.