Why do shot groups move?

by B.B. Pelletier

I couldn’t fit the entire question into the title, so here it is:

Why do shot groups move from side to side when the scope setting doesn’t change?

This question does not include different pellets impacting at different places on a target at the same range, which is caused by the individual flight pattern of each type of pellet. However, I’ll address that question before moving on with the first one.

Today’s question was suggested by Jay in VA, who really wants to know why different pellets or bullets shoot to different places, with respect to left and right. Jay, that’s caused by how each pellet or bullet is stabilized. Back around the turn of the 20th century, Dr. F. W. Mann conducted an experiment in which he locked a barreled action in a 3,000-lb. vice (his “Shooting Gibralter” rest, a concrete pier sunk 40 inches below ground level and extending about 30 inches above, and fitted with a cast iron lockdown rifle rest) and then fired the “gun” at 100 yards in still air. After each shot, Dr. Mann rotated the barreled action 90 degrees, until he had shot it in a complete rotation. His lockdown mount was constructed to allow this rotation, so the barrel was never unlocked from the rest. read more