[Raellus]

By reboring the barrels, you'd also probably not be able to chrome them, which, over the lifetime of the weapon, would contribute to faster wear and corrosion, especially if substandard propellants are being used. I suppose one could chrome them, but that would mean increasing the diameter of the bore even more (albeit ever so slightly). First, this would be quite tricky, and doing so would further weaken the barrel.

It would probably take a long time for a small-scale gunsmithing operation to rebore and rechamber significant numbers of AK-74s. Most of the well-respected companies that assemble former East-bloc AKs here in the States do so in relatively small batches and, AFAIK, they don't have to do anything quite as complex as modifying the bore/chamber. As I understand it, they manufacture a few of the components and then mate them with imported parts (some companies then trick them out quite extensively). Apparently, not every bit of the foreign made, military-grade AK is legal to import and/or sell here; obviously, the fully-automatic capability has to be dropped but there could be other required mods as well. There are waiting lists for online purchases. But, there's not a lot of machining or remanufacturing of the major components done before the rifles are assembled. If reliable power was an issue, this sort of remodding would take even longer. This all was true a couple of years ago, at least. I was researching the purchase of a mostly Polish Tantal AK-74 clone (I didn't go through with it because of the complexity of the transaction). I settled on a AR-15 instead.

I can see why the PRC would want to get some cash from captured rifles, but they could probably put them to use themselves. In the v1.0 timeline, most of the PLA would still be equipped with domestically produced 7.62mm x 39 AK-47 clones but at some point in there, IRL at least, they began transitioning to a smaller caliber rifle round (5.8mm IIRC). Perhaps the Chinese would have issued their captured stocks of 74s to reservists, militia, paramilitary forces, or even SOF.

Maybe the U.S. did buy a bunch of captured AK-74s from China but political/legal wrangling meant that they never hit the civilian market. Maybe they sat in a couple of shipping containers in impound somewhere. Maybe they were stopped and impounded just on the Nevada side of the Cali-Nevada border, and stayed there in legal limbo until the U.S. got dragged into WWIII. Getting ahold of a few hundred or thousand AK-74s would make a cool mission for a CONUS-based game, especially if someone else (another local warlord, the U.S. military, Mexican elements) was also trying to get a hold of them.

Lastly, and I know this has already been mentioned, but it seems like it would be waaaay easier to manufacture 5.45mm ammunition for AK-74s than to rechamber/rebore etc. the rifles to fire 5.56mm. IRL, 74 clones are on the domestic market today and ammo for them is available. I'm not sure where it all comes from (I suspect a lot of it is imported from Eastern Europe) but I don't doubt that at least some of it is manufactured here in the States.