Yep, it's been done before. You might look for something called the 30 Kern. It's basically what you're looking for, designed for cast bullet benchrest matches. Another one that works well in the AR is a 6.8 SPC case run through a 30 Herett die. You can use 6.8 SPC mags, and they're pretty reliable. And there is that outfit that adapts AK mags to AR-15 lowers. Never dealt with them, but it looks like it'd work.
