Bloodline 11.5″ M4 5.56 NATO: This Rosco 11.5″ M4 5.56 NATO Bloodline Barrel is designed to be the ideal addition to any SBR or pistol build. ALL NFA RULES APPLY! Rosco 5.56 (NATO) barrels are made to meet the needs of modern carbine shooters. Rosco is a long way from your daddy’s Sporter AR, and those specs simply won’t suffice anymore. From properly managing dwell times for SBRs, to 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 appropriate for each barrel type.