What causes a bullet to become dynamically unstable?

[flyfisherman246]

So a bullet leaves the muzzle with plenty of gyroscopic and dynamic stability (assuming supersonic and a proper twist rate for the bullet). As velocity slows, the overturning force of air resistance lessens at a much greater rate than the RPM's meaning the gyroscopic stability increases. At some point the bullet becomes dynamically unstable while still having plenty of gyroscopic stability. What causes this eventual dynamic stability down range?

 

[_Windrider_]

The bullets center of pressure moves behind its center of gravity. It’s the same thing that happens to airplanes as their weight changes as fuel burns. This cause the instability and is usually seen as tumbling by the bullet in subsonic flight. In airplanes we can compensate for this at lower speeds by changing the surface area of the wings (flaps, slats) and adjusting AOA. Can’t do that with bullets.

 

[flyfisherman246]

The bullets center of pressure moves behind its center of gravity. It’s the same thing that happens to airplanes as their weight changes as fuel burns. This cause the instability and is usually seen as tumbling by the bullet in subsonic flight. In airplanes we can compensate for this at lower speeds by changing the surface area of the wings (flaps, slats) and adjusting AOA. Can’t do that with bullets.

That’s the part I was missing. Makes sense. Thank you!

 

[Mk16.5]

How does AC change with speed? I've never considered a projectile as a wing. I realize drag decreases inversely with lower speeds, but please expound a little..

 

[alamo5000]

The bullets center of pressure moves behind its center of gravity. It’s the same thing that happens to airplanes as their weight changes as fuel burns. This cause the instability and is usually seen as tumbling by the bullet in subsonic flight. In airplanes we can compensate for this at lower speeds by changing the surface area of the wings (flaps, slats) and adjusting AOA. Can’t do that with bullets.

I was going to say something similar. The balance of the bullet is built only in one location. For example a bullet might have maximum dynamic flight abilities at between 2400 and 2900 fps. As it slows down those dynamics change. You also have external forces too. Shock waves from going transonic or being pushed by the wind are all factors. The slower the bullet the more influence the wind can have on it. The bullets tend to crab walk as the wind blows but the slower they go the less they can resist external forces. They are not flying perfectly level either. They have their own trajectory. When the overall situation gets bad enough the bullet can go butt over nose while it's on it's descent or whatever. Some bullets can withstand a whole lot more than other bullets so none of that is equal across the board.

Some bullets would be extremely stable as they started to coast along back into a descent to earth. Others though might have drag models that do not take that into account. Round nose flat base bullets for example might be extremely accurate out of a given gun when shooting groups at 100 yards but as you stretch things out beyond that the turbulence in the wake of the bullet can cause it to tumble.

This is serious science territory for sure. There is not a single answer. I personally think it would have to be applied to a given bullet in a given situation to explain it. That and a few million dollars worth of scientific equipment.

This is the stuff Bryan Litz does for work.

 

[alamo5000]

How does AC change with speed? I've never considered a projectile as a wing. I realize drag decreases inversely with lower speeds, but please expound a little..

AC?