Judging whether a miss is down to spin drift
PUBLISHED: 11:32 08 February 2017 | UPDATED: 11:32 08 February 2017
Why would rounds be striking six inches to the right with a good zero at 100?
Q: I’m shooting a Blaser R8 with match barrel 1:10 twist, shooting factory Norma 180gr Nosler Accubonds. At 400m with no wind values, why would rounds be striking six inches to the right with a good zero at 100. Is it spin drift?
ANDREW VENABLES replies: Spin drift in .338 is worth about two to three clicks or 20-30cm at 1,000m with 300gr ammo and a 1:10 twist. I doubt it is spin drift in a .308 at 400m. I generally find there is hardly ever ‘no wind’. The merest zephyr of a breeze will move rounds off at ranges like 400m.
That is why groups at these ranges are almost always oval in shape; the windage is the dominating factor. One mile per hour from three or nine o’clock moves a .308 bullet around 14-16 inches sideways at 1,000m, so likely four to six at 400m. One mile per hour is what most people can’t feel and would call ‘no wind’. Accept chaos at ranges over 300m; hunt animals to get within 200m for safe results.