Thread: Ar build

So today I commenced my 1st ar15 build, with careful YouTube guidance it all came together pretty damn easy. Now the dreaded has happened... 1 left over piece. I've been through all the videos and searched parts lists etc, I'm hoping it's been thrown in by mistake. It's the same size as the roll pins except it's a solid piece of steel. All roll pins etc on tutorials went where they should. The gun seems to operate correctly. Any ideas what this is? I'm hoping it's something simple or an extra