So what is China’s real reason for not wanting American citizens to own firearms? None of them shoot far enough to be a threat to China, unless China is thinking about moving over here.
In any conflict between China and the US, one factor is the number of Chinese nationals in the US whose primary loyalty remains with China, and would support China, in a campaign of sabotage and terrorism.
This is much less likely to happen if doing so would result in a campaign of retaliatory attacks by the armed citizenry.