# Spin drift

#### 308175

##### Well-Known Member
Does anybody have any calculations for figuring spin drift?? ANY help would be very appreciated.

#### Rifleman 7

##### Active Member
"Understanding Firearm Ballistics" by Robert A. Rinker: Chapter 9 - Spin-Gyroscopic Drift.

Spin drift is defined by Military Ordinance as, "The lateral deviation of the trajectory from the plane of depature caused by rotation of the projectile." Bullets drift to the right when fired from a right twist barrel and to the left from a left twist barrel. ...it increases at the longer ranges and forms a slightly decreasing radius curve.

There is no practical method of calculating drift from factors such as velocity, range, etc. Various bullet characteristics that cannot accurately be predicted are involved. Consequently, a direct measurement during testing is the only certain method of determining drift.

#### TiroFijo

##### Active Member
The spin drift is due to precession from the angle of repose. The equilibrium yaw for a RH barrel is a little to the right and up.
The drag forces then act up and left and by precession move the bullet a little to the right (and trajectory also "flattens" a little bit downrange).

From a SOTIC instructor:
"As an example, the 7.62mm spins right about .5 moa at 700 meters, .75 at 850 m and 1 at about 925 m.
This is for M118 LR (308 - 175 SMK @ 2680 fps) and and would work for most rounds running about 168 to 175 gr at 2600 to 2700 fps. The heavier and slower the round the more pronounced the spin drift. An example is the old Enfield 303 form Britain in the early part of the century. Spin was 1 moa for each 100 yards. My .50 cal Black Powder rifle spins 7moa at 100 yards and off the chart at 200 with my competition load but the spin is less then half that with my hunting load. The difference is the muzzle velocity."

From Rinker's book:
"The gyroscopic drift of the 30-06 is about 13" at 1000 yds (...is this one of the 150 or 172 gr loads?)".
For ranges below 1000 yds it is suitable to use the relationship of the squares of flight time, or to the third power of the range. At 500 yds it would be 1/8 that at 1000 yds, or 1.5". At 300 yds it would be 1/37 of 13", or 0.35"."

There are a couple of interesting threads regarding this topic here, please do a search.

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#### Guest

##### Guest
Anyone interested in this topic might want to search for the topic Yaw of Repose and read the previous discussions. I gave a simple squared function that approximates Y.O.R. for a big 30 cal shooting bullets between 200 and 220 grains.

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