The effect of increasing metal fraction on carbon arc nanoparticle production is examined for 10–50 weight percent cobalt starting materials. With 500 Torr of helium buffer gas, the carbon arc process yields carbon-coated FCC Co nanoparticles of similar sizes throughout this range. The saturation magnetization is believed to scale linearly with the relative abundance of Co. The variation in the coercivity with abundance is small compared to the dramatic changes which can arise from changes in the size of fine particles. The approach to magnetic saturation is more rapid in high abundance samples. This is attributed to interparticle interactions which align the easy axes when the nanoparticles crystallize within interconnected carbon shells. The switching field distribution as a function of Co abundance arises from both rotational barriers and from barriers due to the particle size distribution.