5.56 Sterling 55gr SP 30rds5. 56 Sterling 55gr SP 30rds is a high quality, economical 5. 5645mm NATO load from Sterling Ammunition, featuring a 55 grain soft point (SP) bullet designed for excellent terminal performance on medium game, varmints, and defensive use. Key specifications: Caliber: 5. 5645mm NATO . 223 Remington compatible Bullet: 55 grain soft point (SP) lead core with exposed lead tip for controlled expansion and rapid energy transfer Muzzle velocity: 3,1003,200