When the bullet leaves the barrel, the rifling has imparted spin to stabilize the bullet in flight. Factors to be considered are:
– Over stabilization affects accuracy
– The longer the bullet the faster the twist needed
– Excessive rpm can cause the bullet jacket to fail due to centrifugal forces. Thin varmint bullets use this phenomenon to increase lethality.
– At long range the spin imparts drift, a factor beyond 500 yards.
– Bullet stability is affected passing through the transonic region of Mach .8 to 1.2, with short bullets with a flat base being affected less than a long boat-tail bullet.
Bullet Weight VS Twist Rate
The M-16 family of firearms has gone from a 1:12 twist rate and 55 grain bullets to as much as a 1:7 twist rate. The ratio given is turns per lineal distance. A 1:12 indicates 1 turn in 12 inches. A 1:7 is one turn in 7 inches, a much faster rate. Military projectiles have very thick and tough jackets with the new M855a1 having a jacket steel core with zero lead. Very tough and capable. Also, these 62 grainers are good to go in a 1:7 twist rate but with an accuracy cost, though tests have proven that a 1:9 rate would be better. Interestingly enough the Army’s match barrels are 1:8 twist and provide good accuracy with the M855a1 round.
Barrel Twist Rate Calculator
The formula for twist rate is the Greenhill Formula.
Twist = Twist rate, one turn in xx inches
D = bullet diameter in inches
C = 150 for velocities up to 2,800 fps. 180 for velocities over 2,800 fps
L = Bullet length in inches
SG = Specific Gravity of bullet, 10.9 cancels out any lead core jacketed design.
“The density of a material is known as its specific gravity. … Soft point jacketed rifle bullets have a specific gravity of approximately 10.25-10.4. The exact density depends on the ratio of jacket material and lead, and the type of lead alloy being used.” (riflebarrels.com/calculating-bullet-weights/)
Twist = 〖C∙D〗^2/L x √(SG/10.9)
This formula comes close to ideal. The thing about twist rates is that the rates are dependent on the bullets length and velocity. A marginally stabilized bullet at lower velocities can be comfortably stabilized at higher velocities. Longer bullets need faster spin rates.
Transonic Range Bullet Speed
Transonic is the region where bullets transitions from supersonic, through sonic to subsonic velocities. Long bullets with boat-tails that perform well at supersonic velocities can suffer from accuracy problems as the bullet destabilizes. Typically, the Center of Mass is located in the body of the bullet behind the ogive. At supersonic above Mach 1.2 or more the Center of Pressure, where all the forces affecting the bullets migration through the atmosphere, is ahead of the Center of Mass. In the Transonic region the Center of Pressure migrates to the front of bullet as velocity is lost, forward some distance towards the point and ahead of the Center of Gravity or Center of Mass (for our purpose the terms Mass and Gravity are equivalent) and begins to cause the bullet to yaw and destabilize. The further the distance between the Center of Mass and the Center of Pressure the less stable the bullet. Short bullets have the Center of Mass and Pressure close together and is the reason short bullets are not as affected by the transonic region as the G7 long low drag projectiles. Short, flat base bullets weather this storm much better than long, relatively thin high ballistic coefficient low drag bullets with boat-tails bases. Thus, the transonic region, where most handgun bullets experience much of their travel are somewhat less affected. Given most muzzle velocities many handgun bullets start out in the transonic region. Muzzle velocities between 900 to 1352 feet per second comprise the transonic region of Mach .8 to 1.2 at sea level.