6.5mm Grendel

The 6.5mm Grendel is an intermediate cartridge designed by Arne Brennan, Bill Alexander, and Janne Pohjoispää as a low recoil, high accuracy, 200–800 yard cartridge specifically for the AR-15. It is an improved variation of the 6.5mm PPC. Wikipedia
 
Case length0′ 2″
Length0′ 2″
Bullet diameter6.71 mm (0.264 in)
Parent case.220 Russian
Case capacity2.3 cm3 (35 gr H2O)
Rifling twist1 in 8" or 1 in 9"
Neck diameter7.44 mm (0.293 in)