Range Report Limits to Don Miller's stability formula for twist

[dbooksta]

I believe Bryan Litz is the one who put Don Miller's gyroscopic stability formula for determining good twist rates into a convenient spreadsheet, which I happened to find here.

Does anyone know what the valid range of inputs are for that formula? For example, does it apply equally well to both VLD and flat-nosed bullets? And does muzzle velocity range extend reasonably as low as 800fps and as high as 4400fps?

 

[BryanLitz]

Re: Limits to Don Miller's stability formula for twist

Don's formula is 'fitted' around data for modern bullets with long ogives and boat tails. A tangent or secant ogive doesn't have a huge effect on the stability prediction.

It's intended for use at supersonic velocities. Remember, the equation is for estimating gyroscopic stability at the muzzle. It would be a mistake to say: "velocity at 1000 yards is 1500 fps, so I want to use the Miller equation to estimate stability at 1000 yards" It doesn't work that way.

It also won't work (well) for subsonic muzzle velocity.

There's another formula available for estimating stability called McGryo, which is hosted on the JBM site under: Calculations-stability.

McGyro isn't as user friendly because it asks for some parameters that aren't easy to get (like Rt/R ratio), but it is able to estimate stability for subsonic muzzle velocities.

Take care,
-Bryan