I've thought about something similar, such as a .223/5.56 case blown out to use .358 bullets, seated to mag length. Like the Blackout, it could use the same bolts and magazines as a 5.56. Should be an effective round with good killing power.
Dang, I knew I had thought of the .350 Legend, years ago - there's the proof
Pity they apparently won't run in 5.56 magazines
