Price not withstanding, 'cuz I don't know it, acquire a "hydra" lower from MGI. This is a unique and truly inovative lower with interchangable magwells to accomodate different mags. As far as I know, they have a standard AR magwell, an AK magwell, and a pistol cal magwell that fits some kind of subgun mag. Along with this lower, they manufacture an AR upper with quick change barrels for different calibers. If you only want one AR, this one's expensive. If you want ARs in multiple calibers, this system will quickly pay for itself. I've read a few reviews on MGI products; all of them faorable.
I just reread your reply. Since you want an AR to share ammo with your AK, might as well get one that shares mags too.
Keep me posted on your project. Also,
AR15.COM has a collective wealth of knowledge on both the AR and AK platforms. I'm
echo_5 on that board.