AR action with AK round?

The AR action (designed by Eugene Stoner) is considered more accurate (but with a somewhat small 5.56mm round), while the AK round is considered more potent (at least out to 300 yards or so), without involving excessive recoil for auto fire (as in the ARs firng 7.62x51mm NATO rounds).

So, why not chamber an AR for the AK 7.26x39mm round?

It's been done. Colt did it at one time.

Cobb Manufacturing offers it as an option.

And Knights Armament has produced the SR-47 (Stoner Rifle 47), which uses standard AK-47 magazines (notice the angled receiver to make this possible), for SOCOM (in limited quantities). [It is not offered to sell to civilians.]

For deep penetration covert operations, there's an obvious advantage to being able to use ammo and mags found or captured in-country. Of course, any unit running across AK-47 mags is probably "finding" AK-47s as well, and could just use them. They might not be as accurate or flat shooting, but they are more reliable and are more likely to tolerate "dirty" ammo without jamming.

The 7.62x39mm round is not equivalent to the .30-.30 for equivalent bullet weights.