Home Forums Hunting & Shooting Ask The Gunwriters Bullet stability

Cold weather makes it harder for the bullet to stabilize, so you'll have to re-test in the winter. If the bullet is stable during the coldest, driest days in winter, then it'll always be stable in that climate.

The rotational velocity stays fairly constant as the bullet moves downrange, but the forward velocity decreases. The bullet gets more and more stable as it progresses downrange.

I have a hard time believing that colder more dense air makes it harder for a bullet to stabilize. I understand that some powders and primers may act differently resulting in less velocity, but I'd like to read a source on that stability problem. Every airplane I've ever flown flies better the colder it gets.

A plane is relying on the density of air to provide lift. Cold is is more dense than warm air.

A bullet is doing something very different. Rather than pushing on the air and using air density to lift up, a bullet is simply trying to cut through the air with as little drag and turbulence as possible. The less dense the air is, the easier it is for the bullet to travel down range, and the smaller the chance that the air will destabilize it.

From Lilja's website:

"Interestingly, changes in air density also have an effect on the twist rate required to stabilize a bullet. That is, when air density decreases, there is a decrease in the amount of spin required to keep a bullet flying point on. Conversely, when the density of the air increases, more spin is required."

"If a bullet is fired from a barrel that has no rifling, the force exerted on the nose of the bullet by the air will cause it to flip over and its trajectory will be wild.

To overcome this tendency, a bullet is spin-stabilized by rotating it about its long axis. The amount of spin required is determined by the specific gravity, shape and construction of the bullet, bullet velocity, and the density of the air it is going to travel through."

"It is interesting to note that this force, or overturning moment, acting on the bullet nose is greatest when the bullet first exits the barrel. If a certain bullet-barrel combination will stabilize a bullet at the muzzle, the bullet will remain stable for the rest of its flight. Why is this so? As a bullet flies on toward the target and beyond, it is losing velocity rapidly as any trajectory table will show. For example a 68 grain bullet from a 6PPC leaving the barrel at 3150 fps will be going 2786 fps at 100 yards and 2449 fps at 200 yards (ballistic coefficient of .265 and standard metro conditions). The rotational speed of the bullet or its RPM's decreases at a much slower rate. The overall result is a lessening force acting on the bullet nose, and that force is being overcome by a proportionately greater spin rate compared to forward velocity. The down-range bullet therefore is more stable than it was at the muzzle. The only exception to this occurs when the bullet passes through the speed of sound. At that velocity, about 1180 fps, it may lose stability."

http://www.riflebarrels.com/article...s_altitude_temperature_rifling_twist.htm