This Rosco 14.5″ M4 5.56 NATO Bloodline Barrel was built around the M4 platform and is designed as the next evolutionary step in the series. It is perfectly suited for your next patrol or duty carbine build. ALL NFA RULES APPLY! Rosco 5.56 (NATO) barrels are crafted to meet the modern-day carbine shooter’s requirements. Rosco is a long way from your daddy’s Sporter AR, and those specs just won’t cut it anymore. Whether managing dwell times for SBRs, running suppressed, or setting up a balanced patrol carbine, Rosco has incorporated your needs into its engineering. Rosco has achieved this by selecting the right twist rates, gas port diameters, and barrel materials based on the specific barrel design.