Establishing the correct twist rate of a given bullet is complex as there are a number of factors that affect the stability of a bullet, namely:
- Barometric Pressure
It is not the weight of the bullet that makes it unstable – rather a combination of the length and the Specific Gravity of the material.
As a rule of thumb the heavier the bullet from the same material and bullet style the longer the bullet. Lead free bullets are typically longer than lead-core bullets of the same weight, making smaller calibres proportionately more sensitive to bullet length and bullet twist.
At best a bullet manufacturer can give a recommended minimum twist. Here at Viper Bullets all our recommendations are based on a gyroscopic stability (SG) factor of 1.5 using the Miller Twist Rule. This does not mean that if the SG drops to 1.4 the bullet will be unstable. In fact bullets are often stable all the way down to 1.2SG, but if it goes under 1.0 it will definitely become unstable and tumble. An SG of 1.3 is seen as the minimum for most conditions. E.g. if you have a bullet that has a 1.25SG and it is stable up on the Highveld at around 5000 feet (1550m) above sea level on a hot day there is a good chance that the bullet cartridge combination will be unstable down at the coast.
A bullet starting out with a 1.3SG could be stable at 100 meters but out at 300 to 400 meters the Spin Gravity (SG) will fall below 1.0 and the bullet will become unstable and begin tumbling. Therefore it is vitally important to test bullet load combinations at the ranges you intend using them at. Don’t take it for granted that if a load is stable at 100m that it will be stable at 300m as you could be in for a big surprise. Check that the holes through the target are perfectly round. An elongated hole will point to an unstable bullet.
Copper and Brass bullets’ Specific Gravity is lower than the SG of lead, therefore they are substantially longer than lead core bullets for a given weight. Because of this it is accepted practice to shoot weights that are 10 to 15% lighter than equivalent lead bullets. Because of the superior properties of copper with a lighter weight bullet performance is vastly superior compared to their lead counterparts. It is wise to err on the side of faster twists rather than under twist.
Twist needed is mostly about bullet length, not weight. The longer the bullet for any given weight, the faster the twist needed to stabilize it. Flat bases are shorter at any given weight than a boat-tail with similar nose form. They also have a longer bearing surface so the slight cocked angle at which they can be pushed down the bore is smaller than for a same-weight boat-tail. A boat-tail takes time to get clear of the muzzle (We call this muzzle dwell) during which muzzle blast plays off it and tends to increase initial yaw for the bullet to recover from (go to sleep) in flight. That same dwell makes perfect symmetry more critical than it is for a flat base bullet, not only of the bullet but of the muzzle crown around the bore axis.
All-in-all, flat base bullets are just easier to find accurate loads for. Their only drawback compared to boat-tails is lower ballistic coefficient, but that won’t normally matter for the first couple hundred meters at least. Even out as far as 500 meters, some flat base designs score better than some equal weight boat-tails except, perhaps, in stiff gusting wind.