Nope, 6mm BBs can vary a bit (relatively) in diameter depending on brand, off the top of my head we're talking about differences in the 0.0##mm range and always under under 6mm (so all BBs are 5.9##mm-something). Better BB brands are closer to 6mm and more consistent with their brothers and sisters, cheaper BBs are not. According to ILLusion, Guarder's the best when it comes to BBs. Crossman, with their *visible* seams, represent the other end of the spectrum.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."