  High-order harmonic generation (HHG) in the relativistic regime is employed
to obtain zeptosecond pulses of $\gamma$-rays. The harmonics are generated from
atomic systems in counterpropagating strong attosecond laser pulse trains of
linear polarization. In this setup recollisions of the ionized electrons can be
achieved in the highly relativistic regime via a reversal of the commonly
deteriorating drift and without instability of the electron dynamics such as in
a standing laser wave. As a result, coherent attosecond $\gamma$-rays in the 10
MeV energy range as well as coherent zeptosecond $\gamma$-ray pulses of MeV
photon energy for time-resolved nuclear spectroscopy become feasible.
