That was my foremost suspicion but otherwise knowledgeable people, who should know better, keep repeating this tale. My first time using the MK19, I asked if the same prohibition followed as the M2 and the instructor looked at me like I had gentalia sprouting from my forehead.
AK. I wasn't exaggerating when I said an M2 is closer to an autocannon than a rifle.
Far left is .50BMG, third from left is a very close stand-in for 7.62 NATO, AK and M16 rounds are the next to the right.