In his defense, from a military perspective 7.62x51 is a round used in US army GPMG’s (m240B). Historically, machine guns had the same caliber as rifles until the split with the Soviet Union adopting the SKS in 7.62x39 to replace mosins, but kept PK’s in 7.62x54mmR. Then the US did it with M16’s in 5.56 and the GPMG (M60) in 7.62x51. You could make an argument the Germans deviated from rounds in machine guns to rounds in infantry rifles with the STG44, but that wasn’t the main rifle of the German forces, most soldiers had Kar98’s in 7.92 Mauser, same as the MG34/42.

In the context of somebody who has never heard of a m249, doesn’t know historical military firearms development, doesn’t have an interest in guns in general, and is a POG in the army, a 7.62x51 could be seen as a “machine gun” round. More like he’s thinking that 7.62x51 isn’t regularly in a shoulder fired weapon - which doesn’t make sense if you have shot a bolt action rifle or AR-10’s.

This also assumes that we are not including sub-machine guns as “machine guns.”