The 5.45×39mm cartridge is a rimless bottlenecked Intermediate cartridge. It was introduced into service in 1974 by the Soviet Union for use with the new AK-74. Wikipedia
Bullet diameter5.60 mm (0.220 in)
Rifling twist255 mm (1 in 10 inch) or; 200 mm (1 in 7.87 inch)
Case length39.82 mm (1.568 in)
Case capacity1.75 cm3 (27.0 gr H2O)
Base diameter10.00 mm (0.394 in)
Neck diameter6.29 mm (0.248 in)