Hi everyone. New to being here . i'll jump into what I'm up against. I'm Building SP1 or what l call SP2 versain of AR. I have A1 upper no assist Used Heavey Barrel 1/7 twist. My Question is the Barrel Nut or Exstention where bolt locks up to barrel and Index pin is not centered to upper. It is off 1/8 of inch to left of Index noch in upper. has anyone had simular problem? Yes all parts are US military issue. Upper is brand new, barrel and exstention is Used but in very good shape . Bolt locks into it OK but haven't installed it Because, Index pin off the Gas tube and Sight are off. any Suggestions?