I'm actually thinking it was a .30-30, that is if the article didn't just truncate something like .308W to ".30 caliber". Can't expect ballistics knowledge and accuracy from regular articles, so all bets are off, really. And it's a matter of how sure you want to be there is absolutely no suffering. Many, many rounds have the capability to cause catastrophic rupturing of the heart, but guaranteeing instant unconsciousness is a more narrow range.
Then you factor in presumed accuracy, where it's not guaranteed the shooters will hit the heart dead on, so you have to maybe use a more powerful cartridge to ensure that even semi-glancing hits will guarantee unconsciousness, etc. There's a lot that goes into it the stricter you want to be.