Spin drift: introduction
When flying towards a target, our oblong bullets spin at nearly 300,000 rotations per minute to resist tumbling through the air and remain stable on its axis of flight. Although this tremendously fast spin is necessary to make precise and accurate shots at distance, this rotation complicates a bullets trajectory downrange, making spin drift one of the most complex topics to accurately represent in marksmanship. This stabilizing spin not only keeps our bullet stable, it elevates the nose of our bullet away from our rifle bore and above its flight arc, it creates differences of pressure on all sides of our bullet, and it even alters the direction our bullet points in flight towards its direction of spin. This final point, the change in bullet direction on account of its spin, is known as gyroscopic drift, or more commonly as spin drift. Spin drift creates substantial horizontal deflection at distance and becomes a key component of an accurate ballistics solution beyond the opening applications of marksmanship.
Variables
The direction of deflection on account of spin drift is dependent on direction of the barrel rifiling. Most barrels use a right hand twist, creating a significant deflection to the right at distance. In the uncommon scenario in which a rifle utilizes left hand twist rifiling, an equal degree of deflection on account of spin drift would be witnessed to the left.
Although atmospheric conditions do play a small factor in the degree of spin deflection, these are negligible for most manual ballistics scenarios, leaving time of flight and distance as our only other variables to consider.
Relevance
Spin drift is best classified as a secondary variable, producing a much smaller effect than bullet drop due to gravity or (typically) horizontal deflection due to wind. Beyond these two primary variables, spin drift produces our next largest deflection, and should be an integral part of all ballistics solutions in the context of sniper teams. If left unaccounted for, spin drift may produce a miss beyond 800m on its own and earlier if compounded with inaccuracies in other variables.
