Why are so many Afghan and Pakistani Krinkovs chambered in 7.62x39? Are they making them themselves or just rechambering parts kits and battlefield pick ups?

I know 7.62x39 is a much more common round there than 5.45 or even 5.56, and that it works better out of a shorter barrel than either of them. But seriously though, are they just making these from scratch in the Khyber Pass?