  The decay of B mesons can be predicted within the Heavy Quark Expansion as
the decay of a free bottom quark plus corrections which are suppressed by
powers of $1/m_b$. This contribution to Loops&Legs 2024 describes the
calculation of the NNLO QCD corrections to nonleptonic decays of a free bottom
quark including charm quark mass effects. In particular it outlines the
challenges in connection to the computation of master integrals, the
renormalization of the effective operators and the problems which arise from
calculating traces with $\gamma_5$ in $d$ dimensions.
