The Arms Guide
  • Home
  • AR-15
  • Rifles
  • Handguns
  • Reloading Gear
  • Gun Safes
  • Gun News
  • About Us

Long Range Shooting: External Ballistics – Spin Drift

yaw of repose and spin drift

Last Updated on March 17, 2022, Published on June 15, 2016 by Chris 1 Comment

Wondering about what generates the spindrift?  Curious to know more about the yaw of repose? In this article, we will examine the reason behind this behavior of the bullet in flight.

In Bullet Trajectory, we found that spindrift is a deflection generated by the gyroscopic motion of the bullet. The direction of the deflection is toward the direction of the spin, and the amount of deflection is dependent on bullet length, flight time, and, not surprisingly, spin rate. Its effect is noticeable and must be taken into account, starting around 500yds. To make an example, for a 175gr SMK bullet, fired at 2700fps from a .308, with a barrel twist rate of 1:12”, the amount of spindrift would be about 1½ in at 500yds and about 9 in at 1000yds. But why does the bullet’s spin generate a deflection in the trajectory?

As we discussed in the previous article about dynamic stability, when it exits the barrel, the bullet wobbles because of gyroscopic precession. When the wobbling is dumped out, and the bullet is dynamically stabilized, the bullet longitudinal axis no longer points in the direction it’s traveling, but instead has a yaw angle, called yaw of repose (or equilibrium yaw), toward the direction of the spin. The bullet is actually skidding along the trajectory, with the center of gravity following the trajectory and the tip pointed to the right (for righteous spin bullets) and slightly upward. The incoming air pushing on the left side of the bullet causes it to drift.

The yaw of repose should be (I’ve found contrasting opinions about this) another result of the phenomenon of gyroscopic precession. Indeed, when an external force is applied toward the axis of spin of a spinning object, the object doesn’t react in the direction of the applied force, but perpendicularly, at 90° in the direction of the spinning. Once again, we can observe this phenomenon with our spinning top example. If you push the toy on one side while it’s spinning, it won’t react in the direction of your pressure, but at 90° in the direction in which the toy is spinning.

The bullet, as we have seen in the bullet shape article, tends to fly with its axis parallel to the line of departure, forming an angle (the angle of attack) with the axis of movement (that is, the trajectory). This means that the airflow is actually coming from below the projectile. The bullet, behaving like a weathervane, tends to point its nose in the direction of the incoming air. Because of the phenomenon described before, this downward force generates a yaw in the direction of the spin, resulting in the yaw of repose and, consequently, in the spin drift.

In the next article, I’ll talk about a very important long-range shooting topic: the passage of the bullet through the transonic region. The transonic region range is considered by many shooters as the limit range for an accurate shot. If you want to discover the reason, stay tuned!


Filed Under: Handguns

Related Posts

  • Best Ar 15 Slings
    Best 9mm Glock Models Reviewed
  • Armed,woman,with,semi Automatic,in,home,invasion,scenario
    Florida Sheriff Tells Residents to Take a New Approach to Home Invasions
  • Aftermarket Glock Slides
    Best Aftermarket Glock Slides

About the Author

Chris has over 30 years experience as a writer in various technical fields including firearms, automotive, engineering, and biotech. He is thrilled to bring his knowledge to The Arms Guide to help the novice and first time gun owners make educated choices.

About us

Comments

  1. Avatar for Chriswbg says

    July 1, 2016 at 3:00 am

    If the bullet settles into a steady state with the noise pointed slightly off of axis then wouldn’t the drift be from the torque ? Not an easy topic.

Leave a Reply

Your email address will not be published. Required fields are marked *


In Case You Missed It

Judge,gavel,and,gun,on,usa,flag.,gun,law,concept
Gun News

Gun Control Looms After Texas Bloodbath

May 25, 2022

A,gun Free,zone,signpost,on,a,fence.,gun,control,in
Gun News

Nevada Takes a Stand Against Gun Free Zones

May 24, 2022

Supporters,of,the,extreme,far Right,golden,dawn,party,celebrate,after
Gun News

The Real Big Lie About White Supremacy and Gun Violence

May 23, 2022

Berkeley,,ca, ,august,27,,2017:,unidentified,participants,at,the
Gun News

Antifa’s Growing Use of Guns

May 22, 2022

© Copyright 2022 Thearmsguide · All Rights Reserved.

  • Contact Us
  • Privacy Policy
  • Terms of Use
  • About Us

Affiliate disclosure: As an Amazon Associate, we may earn commissions from qualifying purchases from Amazon.com.