That's not exactly what I'd call proof. What might be more convincing is this. If you think about the machine guns added in the DLCs, you've got the BAR which uses the .308. HH adds the .45 Auto SMG. Granted it's not the .45-70 gov't, but it is .45 cal. OWB adds the K9000 and FIDO which are .357 and .44 respectively. From the stock game we've already got 5mm (minigun), 9mm, 10mm, 12.7mm, 5.56mm (light machine gun), .22LR, and electron charge pack.
That means that what is missing a machine gun, is the energy cell, microfusion cell and the .50 MG.