by Brian Young, Captain USN(r) , CSEC LRP Range Officer.
The CSEC LRP Wind Drift Estimator updates and expands upon an old USMC wind drift (WD) thumb rule; Distance divided by 100 (R), times Crosswind (CW), divided by a constant K, which ranged between 10 and 14. WD = (RxCW)/K
CSEC LRP Wind Drift Estimator uses a more direct approach, in that K represents the crosswind speed that produces 1 MOA (.3 mil) bullet drift for every hundred yards at sea level, with a reference bullet weight and muzzle velocity. Corrections to K for altitude, bullet weight, muzzle velocity are provided for several common calibers. Examples for each caliber are provided.**
** Note: The CSEC LRP Wind Drift Estimator is based on high-BC, Spire Point Boat Tail bullets; it is not usable for flat-based round nose bullets, but may yield usable results for flat-based Spitzer bullets, such as Nosler Partition, inside of 600 yds.
To calculate actual Wind Drift, divide the Crosswind (CW) by K and multiply by R, where R is distance in increments of 100 yards.
WD(MOA) = (CW/K) x R
WD(MIL) = .3(CW/K) x R
The CSEC LRP Wind Drift Estimator is precisely that, an estimator, not intended to be exact, but sufficiently accurate to spot hits or bullet splash out to 2000yds. However, the source of largest error will always be the estimate of cumulative downrange crosswind speed. Therefore, learning to accurately estimate crosswind using scope mirage, environmental factors, etc. is an important marksmanship skill.*
Note: Bullet Spin Drift is not included in the estimator and must be applied independently.
The CSEC LRP Wind Drift Estimator’s K for a particular load and altitude can be calculated well in advance of shooting, even days or months before going into the field or to the range, thus simplifying calculations to make wind calls.
Base K: for 6mm K=11, 6.5mm K=15, 7mm K=16, .308 K=10, .338 K=15
Base Alt = 0 feet, sea level
Base MV = 2700fps for all calibers.
Base BW: 6mm = 90gr, 6.5mm = 140gr, 7mm=175gr, .308 = 165gr, .338 = 250gr
Alt correction: (.00006 times altitude feet)
BW correction: ( ±.01 per 1 grain above/below Base BW) MV correction: ( ±.001 per 1 fps above/below 2700fps)
To calculate K, Crosswind speed to produce 1MOA (.3 mil) per 100 yards:
K = Base K x [1+(.00006 x Alt) + ((.01 x (BW- Base BW)) + ((.001 x (MV – Base MV))]
Extreme Long Range Wind Drift Estimator (1200 yards for 6mm; >1300 yards for all other calibers).
Calculate WD for 1000 yds (WD1000) then multiply by R/7.5.
WD(ELR) = WD(1000) x [(R)/7.5]
Examples by Caliber (Click on the link for a PDF file):