6mm Remington

The 6mm Remington rifle cartridge, originally introduced in 1955 by Remington Arms Company as the .244 Remington, is based on a necked down .257 Roberts cartridge using a .24/6mm bullet. Wikipedia
 
Bullet diameter.2435 in (6.18 mm)
Case length2.233 in (56.7 mm)
Overall length2.825 in (71.8 mm)
Base diameter.471 in (12.0 mm)
Neck diameter.276 in (7.0 mm)