The "pion-decay" bump is a distinct signature of the differential energy spectrum of γ-rays between 100 MeV and 1 GeV produced in hadronic interactions of accelerated particles (cosmic rays) with the ambient gas. We use recent parametrisations of relevant cross-sections to study the formation of the "pion-decay" bump. The γ-ray spectrum below the maximum of this spectral feature can be distorted because of contributions of additional radiation components, in particular, due to the bremsstrahlung of secondary electrons and positrons, the products of decays of π±-mesons, accompanying the π0-production. At energies below 100 MeV, a non-negligible fraction of γ-ray flux could originate from interactions of sub-relativistic heavy ions. We study the impact of these radiation channels on the formation of the overall γ-ray spectrum based on a time-dependent treatment of evolution of energy distributions of the primary and secondary particles in the γ-ray production region.