A “hot round” is any round at the top of the powder range for the caliber you are shooting. 45 grains of Varget powder for a 308 round would be considered a hot round. Top it off with a 180 grain bullet and it becomes a “match” round.
And this was a sabot round! A specialty round that would not move down the barrel in sufficient time to relieve some of the over-pressure. Wonder if he got that contraption on “special”?