.458 SOCOM

The .458 SOCOM is a moderately large round designed for a specialized upper receiver that can be mounted on any AR-15 pattern lower receiver. The 300-grain round offers a supersonic muzzle velocity of 1,900 ft/s and 2,405 ft⋅lbf, similar to a light .45-70 but with a much smaller case. Wikipedia
 
Bullet diameter11.63 mm (0.458 in)
Case length40.00 mm (1.575 in)
Parent case.50 Action Express
Rifling twist1 in 14" (most common) or 18"
Base diameter13.74 mm (0.541 in)
Rim diameter12.01 mm (0.473 in)
Neck diameter12.32 mm (0.485 in)