Previous work by the present authors on metal films sputtered from cylindrical-post magnetron sources has established the existence of an abrupt transition in internal stress and other properties that occurs as the working pressure is lowered. High compressive stress, maximum reflectance, minimum resistivity, entrapped inert gas, and dense, Zone-T microstructure were found in a wide variety of pure metals and alloys when sputtered in argon at sufficiently low pressures. The present study reports this same phenomenon for a single metal (Mo) sputtered in a variety of gases (Ne, Ar, Kr, Xe). In this case the transition pressure varies inversely with the mass of the sputtering gas, from less than 0.3 Pa for xenon to over 0.9 Pa for neon. The concentration of inert gas in the films also varies inversely with the mass, increasing by two orders of magnitude from krypton to neon. Deposition through a narrow slit-aperature established that the entrapped gas comes from the sputtering cathode (backscattered, neutralized ions) by line-of-sight trajectories. Moreover, right-angle scattering of gas from the sides of the cylindrical-post magnetron, as viewed by a given substrate, becomes increasingly prominent for the heavier gasses, and is shown to be a principal factor in causing compressive stresses. The evidence indicates that compressive stress and the presence of entrapped inert gas in the films are both by-products of inert gas bombardment of the condensing material during deposition. A significant correlation is found between the variation in transition pressure and the decreased scattering cross sections reported in the literature for energetic inert gas particles, such as those scattered from sputtering targets of greater mass.