On the AR the bbl extension is threaded onto the bbl by the manufacturer and is not normally removed after that. It is made that way as a manufacturing expedient, you can machine the locking lugs and such a lot easier witrh access to both ends. The smooth exterior of the extension is a slip fit in the AR upper, there is also a locating pin and a slot in the upper to align the two components. A large nut then threads over the bbl mounting area of the upper, retaining the bbl. Don't have the dimensions handy, but the Ar bbl is so big at the rear I doubt you would have much trunnion left after drilling it out to fit the bbl.
If you really wanted to use the Ar system in an AK, you could cut off both an AK carrier and an AR carrier and weld the two together, then use the stock AR bolt.