6mm Optimum

The "6mm Optimum" is a concept popularized in 1999 by military writer Stanley C. Crist.

History
Crist argued for the adoption of a 6mm rifle chambering as a replacement for the venerable 5.56×45mm NATO and 7.62×51mm NATO, calculating that such a loading would be near the weight and minimal recoil of the lightweight 5.56 while nearer the terminal performance of the heavier 7.62, thereby permitting a single caliber to serve in general-purpose machine guns, sniper rifles, and infantry carbines.

Ballistics
Crist's specifications were that "6mm Optimum" ammunition achieve, with a 100 grain bullet: ...of which he noted, "...even with a conservative estimate for the muzzle velocity of the 6mm Optimum cartridge, computed data for 1200-meter velocity, flight-time, wind-deflection, and trajectory height are all greatly superior to both 5.56 and 7.62 NATO rounds."
 * velocity: 2,900fps (muzzle), 1149fps (1,200m)
 * energy: 1,867ft-lbs (muzzle), 293ft-lbs (1,200m)
 * flight-time to 1,200m: 2.21 seconds
 * deflection @ 1,200m in 10mph crosswind: 151 inches
 * maximum trajectory: 244 inches