Small parts help

I bought a FDE 2.5S but didn't even notice it was missing a bolt which I think hold the barrel to the receiver. I've been researching looking at old posts and I think I'm SOL here anyone know what size bolt it is or where I can get replacement?

I bought a FDE 2.5S but didn't even notice it was missing a bolt which I think hold the barrel to the receiver. I've been researching looking at old posts and I think I'm SOL here anyone know what size bolt it is or where I can get replacement?