I think you might be starting from the assumption that the Chinese Civil War and fighting the Japanese was done with privately owned firearms. It wasn't.

There were several arsenals for the Chinese government that used Western Weapons(dating back to the Qing Dynasty). One of the most common rifles was the Hanyang 88, a copy of the Gewehr 88. In fact, some of the arsenals even made nonsense words with the Roman alphabet and fake Mauser corporate proof marks to make them appear as if they were made in Germany(much the same way that some Khyber Copies of AKMs have fake Russian arsenal stamps).

The other major source during this time frame was from foreign suppliers, either paid for or gifted by Allies like the US. This meant the weapons were given to the Nationalist government, not to random schmoes.

With that in mind, many of those firearms remained in more or less positive control of the Government/factions that had those armed forces. The government(both Mao's forces and Chiang Kai-shek's forces) "disarmed" the population the same way that other countries do to returning soldiers: When you check out you return your firearm to the armory or otherwise have it accounted for.

A follow up question to this: what happened to the guns that people were armed with during the Cultural Revolution? I'm under the impression that Mao favored arming radical workers and students at that time