There’s a lot of good guns in the Outlaw, a lot of good, American guns. (5400 manufacturers and 310 million weapons, remember.) So how did a Chinese variant of a Soviet WWII assault rifle become the weapon of choice for a lot of good Americans in 2039?
The Chinese. They had the money, and the need.
When the fleeing Communist Party arrived in Xiyatu (Seattle) in force, they brought with them a (literal) boatload of Type 56 rifles and lots of ammo. A mistaken incursion into Vancouver territory (in 2019) started a long-running, low intensity conflict with the recovering Canadian state, which ate into their ammo reserves. Plus, there was the usual incursions from the various gangs roaming the Outlaw (though it wasn’t yet called the Outlaw).
The mainland was still fighting a vicious civil war, so trade was nonexistent. And American supplies of the 7.62mm round were scarce, to say the least. So the Chinese turned to the next best thing: a still-extant (and expanding) American arms manufacturer.
Utah had avoided much of the violence of the Black Year (the year of famine and disease after the Rotting Plague, 2015-2016). They had enough food on hand to feed their people (when redistributed by the short-lived United Order), their economy was still functional, if only just, and they even had domestic sources of oil in southern Wyoming and refineries in the state capitol of Salt Lake City. Browning Arms (in Morgan, Utah) was making guns and bullets as fast as they could.
See, the word spread that Utah had food and gasoline. A great many packs and gangs thronged the state. The packs were taken care of as best as the state government could manage, but the gangs were dealt with harshly. This took weapons and ammo, which Browning could provide. So when the Chinese went shopping for ammo (in 2020), Browning was pretty much the only potential source. If they could be convinced to begin production.
The Chinese had escaped with billions in bullion and other valuables. They needed a steady supply of ammunition, so they contracted with Browning to provide 7.62mm rounds on a permanent basis, in exchange for gold (US and Chinese tender being useful only as tinder at that point). On the strength of that contract, Browning expanded their operations.
Browning took over a small building in Morgan (that used to be an Ace hardware store, before the Collapse), and began making 7.62mm shells. As time went on, they expanded their operation, even mechanizing it (to an extent). In the years to come, they would also begin manufacturing replacement parts for the Type 56 rifles and even assembling new ones for sale (they had the parts just lying around…).
Post-Collapse, violence was common and self-defense a necessity. People needed guns. AK’s are very easy to manufacture, and consequently cheap, so when presented with a rugged, effective, and cheap weapon, many people opted for the Chinese variant of a Soviet assault rifle, even if they were American.
The AK-47 was adopted for practical, identifiable reasons. But its first appearance in the Outlaw, and its ubiquity, was entirely due to the Chinese.