I had a .450 Bushmaster for a while then gave it away. But I liked it so much I will probably get another. I had the carbine version. For a big AR the carbine was balanced very well. It also carried nicely - not heavy, easy in the hand, and comfortable on the shoulder or back. Recoil was not unpleasant because of the gas system and buffer assembly. Accuracy was excellent.

The .450 Bushmaster shoots .452 caliber, .45 pistol diameter, bullets. Two top ammo manufacturers make cartridges for the .450 - Hornady and Remington. To see what others think of the factory ammo you can read some reviews: http://www.midwayusa.com/features/quickreviews/979384

Choosing an AR because it shoots .458 rifle bullets over one that shoots .452 pistol bullets makes little sense. The .458 SOCOM was designed for use by SOCOM (Special Operation Command) where mushrooming of bullets isn't in the equation. It would be your best big bore AR choice for barrier penetration and use against personnel with heavy body armor. There are many .458 bullets available but most of them are heavy and made for shooting from magnum rifles. Bullets made for the .45-70 are probably the best .458 option for shooting at AR velocities. However you will find a wide array of .452 bullets in weights of 250gr, 260gr, 275gr, 300gr, and 325gr that will perform properly on game when shot at .450 Bushmaster velocities.