From Guns.com:

Missouri-based CMMG on Tuesday announced the first firearm made for the American consumer market chambered in 4.6×30 – the FourSix.

Designed originally by Heckler & Koch in the 1990s for a low-recoiling NATO Personal Defense Weapon program – competing against FN’s 5.7×28 for the same purpose – the 4.6x30mm is a rimless bottlenecked cartridge that performs well under 150 yards with a small size that allows guns chambered for it to carry a lot of rounds.