Way too many variables in the system to be certain what causes that sort of thing... but it is NOT the bullet suddenly going unstable or wandering as a function of the bullet.
Among the suspects would be parallax in the sighting system, number one.
A poor fit between reticle and target at the longer range can create issues both because of the physical characteristics of the optics and target and because of eye variations in different people.
Unseen weather at the longer distance creating air movement...
And many more, but bullets are not the problem.
I am way more of a hunter than a shooter, and certainly am no physicist, so I honestly have no idea why my experience with them is what it is.
I don't think parallax or different wind conditions have been the issue...as other bullets out of the same rifles and scopes at the same range sessions have grouped just fine at distances way beyond where the Sierras (and Noslers) would start going to hell. Why Sierra bullets do this escapes me and I will bow to others with more shooting experience, but I do believe it is the bullets, for whatever reason.
