The 5.56-millimeter M27 is based on Heckler & Koch’s HK416. It’s piston-driven and sports a free-floating barrel. Marines reportedly like the M27 for its reliability and accuracy. A marksman’s variant called the M38 boasts a Leupold TS-30A2 Mark 4 MR/T variable-power scope in place of the Marines’ standard ACOG sights.