  Precise measurements of $b\to c\tau\bar\nu$ decays require large
resource-intensive Monte Carlo (MC) samples, which incorporate detailed
simulations of detector responses and physics backgrounds. Extracted parameters
may be highly sensitive to the underlying theoretical models used in the MC
generation. Because new physics (NP) can alter decay distributions and
acceptances, the standard practice of fitting NP Wilson coefficients to
SM-based measurements of the $R(D^{(*)})$ ratios can be biased. The newly
developed HAMMER software tool enables efficient reweighting of MC samples to
arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how
HAMMER allows avoidance of biases through self-consistent fits directly to the
NP Wilson coefficients. We also present example analyses that demonstrate the
sizeable biases that can otherwise occur from naive NP interpretations of
SM-based measurements. The HAMMER library is presently interfaced with several
existing experimental analysis frameworks and we provide an overview of its
structure.
