I'm sure this probably wasn't "it", but just as FYI - Armalite has made some "special" M4's for Spec Ops guys searching caves in Afghanistan that shoot 7.62x39, AND takes AK47 mags! They said it was impossible to hump enough ammo to do the job, and they'd sometimes be in caves FULL of 7.62x39 ammo and AK mags.
Looks odd to say the least.
I tried to find the link again, but couldn't.