  We consider the (numerically motivated) Nanbu stochastic particle system
associated to the spatially homogeneous Boltzmann equation for true hard
potentials. We establish a rate of propagation of chaos of the particle system
to the unique solution of the Boltzmann equation. More precisely, we estimate
the expectation of the squared Wasserstein distance with quadratic cost between
the empirical measure of the particle system and the solution. The rate we
obtain is almost optimal as a function of the number of particles but is not
uniform in time.
