The AK-47 uses 7.62x39, while the M-14 / M1A etc uses 7.62x51 (i.e. .308). Not the same ammo.
The Saiga .308 uses 7.62 Nato and can have high capacity magazines, it is made at Izhmash.... same place they make the ak 100 series
It does raise a mass production question, though. I imagine it would be simpler to retool between x39 and x51, than with 5.56. I also suspect that the tolerances for 5.56 are very tight compared to 7.62.