Bloodline 10.5" M4 5.56 NATO: This Rosco 10.5" M4 5.56 NATO Bloodline Barrel is designed to be the perfect addition to any SBR or Pistol build. ALL NFA RULES APPLY! Rosco 5.56 (NATO) barrels are designed to meet the modern day carbine shooters requirements. Rosco is a long way from your daddy's Sporter AR, and those specs just won't cut it anymore. From properly managed dwell times for SBR's, to running Suppressed, or setting up a balanced patrol carbine Rosco has incorporated your needs into Rosco engineering. Rosco has successfully accomplished this by picking the right twist rates, gas port diameters, and barrel materials according to the barrel being made.