View Full Version : Bullet stability

11-10-2009, 12:53 PM
I remember seeing in a recent thread somewhere (can't remember if it was this forum or not) about bullet stability and the factors that go into calculating an exact number. The reason I'm curious about it is because I'll have a Suhl 150-1 in my hands later this week and am aware that the twist rate in those barrels did vary some, which means the temperature will be even more important in figuring out how stable that bullet will be in a given condition.

Because I live (and shoot) in Minnesota, it's pretty obvious that the general temperatures are much cooler than other places and I am concerned that I might have just bought a rifle that won't shoot well unless it's rather warm out. Can anyone give me some insight into this or get that equation back out?


11-10-2009, 05:53 PM
Don Millerís twist equation is probably the best method to calculate Sg (Projectile Stability) but there will be much more variance in predicting the correct stability for lead rimfire bullets because they will not keep the same shape after pressure, acceleration, and the bore itself changes the original configuration of the bullet.

If you have Microsoft Excel, PM your email addy and I can send a spreadsheet with the equation.....or you can do it yourself:

Sg=(30*Projectile mass in grains)/((Twist in inches per turn/Projectile diameter in inches)^2* Projectile diameter in inches ^3*Projectile length in inches/ Projectile diameter in inches *(1+( Projectile length in inches / Projectile diameter in inches)^2))*(Muzzle Velocity/2800)^(1/3)*((Temperature in Farenheit+460)/(59+460)*29.92/Pressure in inches of mercury)