  Within the TMEP, we present a detailed study of the performance of different
transport models in Sn+Sn collisions at $270A$ MeV, and put particular emphasis
on the production of pions and $\Delta$ resonances, which have been used as
probes of the nuclear symmetry energy. We prescribe a common and rather simple
physics model, and follow in detail the results of 4 BUU models and 6 QMD
models. The nucleonic evolution of the collision and the nucleonic observables
in these codes do not completely converge, but the differences among the codes
can be understood as being due to several reasons: the basic differences
between BUU and QMD models in the representation of the phase-space
distributions, computational differences in the mean-field evaluation, and
differences in the adopted strategies for the Pauli blocking in the collision
integrals. For pionic observables, we find that a higher maximum density leads
to an enhanced pion yield and a reduced $\pi^-/\pi^+$ yield ratio, while a more
effective Pauli blocking generally leads to a slightly suppressed pion yield
and an enhanced $\pi^-/\pi^+$ yield ratio. We specifically investigate the
effect of the Coulomb force, and find that it increases the total $\pi^-/\pi^+$
yield ratio but reduces the ratio at high pion energies, although differences
in its implementations do not have a dominating role in the differences among
the codes. Taking into account only the results of codes that strictly follow
the homework specifications, we find a convergence of the codes in the final
charged pion yield ratio to a $1\sigma$ deviation of about $5\%$. However, the
uncertainty is expected to be reduced to about $1.6\%$ if the same or similar
strategies and ingredients, i.e., an improved Pauli blocking and calculation of
the non-linear term in the mean-field potential, are similarly used in all
codes.
