7.62mm NATO - 9.5 g, 852 m/sec, 8.1 kg m/sec
3" Magnum rifled shotgun slug - 28g, 536 m/sec, 15 kg m/sec
Hence, the most powerful rifled shotgun slug I can find has about double the momentum at the muzzle of a 7.62mm NATO cartridge. Since as a cartridge this is 50% longer and weighs about three times as much, I am mildly peturbed that anyone would find this a massive surprise.
It should also be noted that a rifled shotgun slug is only travelling at 298 m/sec at 100 yards, giving it a momentum of 8.3 kg m/sec - the NATO round in comparison will barely have slowed down at this range. In other words, the improved momentum transfer of a rifled shotgun slug is only valid out to about 100m.
I’ve no idea why anyone cares though - it’s completely irrelevant outside a coconut shy!