I’ve previously posted about the importance of matching projectile weight to your barrel’s twist rate, but today’s topic dives deeper into another crucial factor projectile length. Many shooters focus solely on weight when selecting bullets for their rifles, but in reality, length plays an equally, if not more, important role in bullet stabilisation.
For a projectile to maintain a stable flight path, it must rotate at a sufficient speed to generate gyroscopic stability. Think of it like a well thrown football without enough spin, it wobbles through the air, veering off course.
If a bullet lacks enough rotational speed, aerodynamic pressure at the tip can cause it to destabilise mid flight, leading to erratic performance, tumbling, or even keyholing when it hits the target. This is why barrel twist rate is so important it determines how fast the rifling imparts spin on the bullet.
While bullet weight does contribute to stability, it’s actually the length of the bullet that dictates how much spin is required to keep it stable in flight. This is because the further apart the bullet’s center of gravity (CG) and center of pressure (CP) are, the more it wants to pitch and tumble if not properly stabilised.
- Shorter bullets are easier to stabilise because the distance between the CG and CP is small, meaning less torque is needed to keep the bullet aligned in flight.
- Longer bullets (such as modern high BC projectiles used in long range shooting) have a greater distance between these two points, requiring more spin to prevent them from flipping end over end.
Long range shooting has seen a surge in the use of long, high ballistic coefficient (BC) bullets that cut through the wind more efficiently. While these bullets offer superior wind resistance and better energy retention at distance, their increased length means they demand faster twist rates to stabilse.
For example:
- A 168gr .308 Sierra MatchKing (relatively short for its weight) stabilises fine with a 1:12 twist barrel.
- A 230gr Berger Hybrid .30 cal (much longer despite only a 62gr weight difference) requires a 1:9 or even 1:8 twist to stay stable.
Range Essentials
5.11 Rush 24
Pelican iM3100
The weight difference alone does not tell the full story, the longer bullet has greater aerodynamic forces acting on it, requiring more rotational stability to keep it pointed in the right direction.
To ensure proper stabilisation, you can use the Miller Stability Formula or an online twist rate calculator. A general rule of thumb:
- If your stability factor (SG) is below 1.3, the bullet is not stable.
- If SG is between 1.3 and 1.5, the bullet is marginally stable but may struggle at longer distances.
If SG is above 1.5, the bullet is well-stabilised and should perform consistently.
Projectile weight alone doesn’t determine stability, length is just as important. Longer bullets require a faster twist rate to prevent them from tumbling in flight. Modern long range bullets are optimised for aerodynamics but demand proper barrel matching to take full advantage of their high BC. Before selecting a bullet, check its stability factor using a ballistic calculator or Miller Stability Formula to ensure your rifle can handle it. When handloading for long range precision, always consider both weight and length to find the optimal projectile for your barrel’s twist rate, this will maximise accuracy, consistency, and downrange performance.
PRECISIONRIFLE