Cobb Manufacturing did indeed make a modular AR system that shot 30/06,300WM and other calibers for a while.
They had financial problems and sold out to Bushmaster,which sold out to Remington.
Cobb MCR Multi Caliber Rifle
The 458 SOCOM shoots .458 caliber rifle bullets,and the 450 Bushmaster uses .452 pistol bullets.
You have a larger choice of bullets to choose from in the 458,and that's why I went with it.
There is factory ammo available for both,but it is very expensive,and most people that have them load their own ammo for these calibers.