RPMC simulation with the runaway collision rate in Eq. (93), a dimensionless threshold mass of m_th = 10⁷ (indicated by dotted red horizontal line in (b)), a total mass of M = 10¹⁰, a homogeneous swarm mass distribution, a particle number threshold of N_th = 20, and n = 2500 RPs. Every individual swarm i has mass M_i = M/n; particles are resolved individually once they grow beyond the mass M/(nN_th) (indicated by the dashed orange horizontal line in (b)). Each panel shows a time series of histograms. The mass-weighted particle number density is shown on the vertical axis of (a), whereas in (b) the mass-weighted histogram bin counts are colour-encoded on a logarithmic scale, and the mass of the runaway particle is shown separately (red curve). Histogram bin counts and mass of runaway particle are averaged over 10 runs, the error bounds of the runaway particle being indicated by red dashed curves.