Barrel whip happens when the barrel vibrates up and down and in a circular motion when being fired... Although there are some up and down vibrations, the main vibrations are circular... If this were not true, then a 3 shot group from a rifle would always be in a vertical string... Most 3 shot groups you will see will be virtually triangular in shape, this is caused because as the barrel vibrates through its "circular arc" one bullet leaves the muzzle at say 12 o’clock, another at say 4 o’clock and the third at maybe 8 o’clock... The larger the arc of the barrel, the less accurate the rifle will be, and the larger the triangle... As a rule the less mass a barrel has (the thinner), the more it is affected by the vibrations, this is the reason that a "heavy" barrel seems to shoot more consistently than a thinner barrel...
Builders of accurate rifles agree if the barrel's movement can't be eliminated, the next best thing is that it move consistently with each shot... That's why good shooting rifles have stiff "heavy" barrels and their actions are firmly bedded in the stock with nothing touching the barrel in front of the receiver (free floating)...