  Two kinds of spectral control over $\gamma$-ray echo using a nuclear
frequency comb system are theoretically investigated. A nuclear frequency comb
system is composed of multiple nuclear targets under magnetization (hyperfine
splitting), mechanical motion (Doppler shift) or both, namely, moving and
magnetized targets. In frequency domain the unperturbed single absorption line
of $\gamma$-ray therefore splits into multiple lines with equal spacing and
becomes a nuclear frequency comb structure. We introduce spectral shaping and
dynamical splitting to the frequency comb structure respectively to optimize
the use of a medium and to break the theoretical maximum of echo efficiency,
i.e., 54\%. Spectral shaping scheme leads to the reduction of required sample
resonant thickness for achieving high echo efficiency of especially a broadband
input. Dynamical splitting method significantly advances the echo efficiency up
to 67\% revealed by two equivalent nuclear frequency comb systems. We also show
that using only few targets is enough to obtain good echo performance, which
significantly eases the complexity of implementation. Our results extend
quantum optics to 10keV regime and lay the foundation of the development of
$\gamma$-ray memory.
