RPMC simulation of the grains-and-boulders model introduced in Sect. 5 for different boulder–grain particle mass ratios for comparison with Fig. 7: (a) total number of grains N_g; (b) average boulder mass 〈m〉_B. The simulation uses n = 1 792 RPs divided into n_B = 256 boulder-mass RPs and n_g = 6n_B = 1 536 grain-mass RPs with equal-weight swarms, M_i = M_j ∀i, j ∈ {1, …, n}, and an initial number of grains. The boulder masses in (b) are averaged over 1 consecutive runs; the filled area indicates the physical spread of the mass distribution, and the error bars indicate the standard deviation of the mean boulder mass over 1 runs. The horizontal black line indicates the expected final boulder mass .