Twist rate /stability calculator

S

SJCbklyn

New member
Berger has a twist rate stability calculator that I have been messing with. Basically like a ballistic calculator but input your barrel twist rate and it will tell you about a particular bullet stability coming out of your barrel.

The more I use it, the more I question its sanity.

Wouldn't a tool like this need to know barrel length or is that insignificant in calculating bullet stability?
 
Lots of factors play into the whole bullet stability thing. Biggest factors would be bullet length and barrel twist. Velocity is important but what it took for you to achieve that velocity matters not.
 
Why would barrel length matter? A 1:10 twist is going to spin the bullet the same regardless of barrel length.
 
As brasscollector says, muzzle velocity is an input for the stability calculator I use. The barrel length does not figure in except as it generates velocity.

Trivia: One teeny exception is the Italian Carcano. It has gain twist rifling and back in the day of cheap surplus, people who cut the barrels short and handy were disappointed with inaccuracy or even keyholing.
 
Barrel length doesn't matter as long as it efficiently burns the powder.

Meaning the closest you can get to completely use the powder to push the bullet, yet be burnt as the bullet leaves the barrel.

Velocity does not mean accuracy. You need to reach a compromise between accuracy and a bullet design that works at range of your target.


To accomplish that, you need a stable bullet in flight, meaning the correct spin for the length of the bullet.

Its really that simple.
 
Barrel length matters in a small way, whether it is meaningful to you or not really depends on if you are building something non-standard in the firearms world.

A 14.5" barrel with a 1:7 twist shooting a bullet at 3,000 fps is going to be just over 304,000 rpm.

A 24" barrel with a 1:8 twist shooting a bullet at 3,400 fps is going 304,000 rpm.

Muzzle velocity (in feet per second) times 12 inches per feet divided by twist rate (in inches) time 60 seconds per minute = bullet RPM.

Sometimes, you can use a slightly slower twist in a longer than expected barrel to get stability from a bullet that is unstable in a shorter length barrel. Sometimes.

But as far as any of the general stability calculators go, as long as it gives you a "stable" you should be just fine shooting from your rifle. The standard calculators weren't meant to help the custom built AR-15 space gun with a 1:9 twist launching 75gr Amax over a max charge from a 26" barrel.

Jimro
 
The unknown in Berger twist rate stability calculator is the velocity,altitude and temperature. All three of those will effect twist rate.

Range I shoot at is over 7K and Berger calculator would change if I was at sea level.
 
How's that unknown? It's right there in the parameters for you to enter into the calculation


Sent from my iPhone using Tapatalk
 
"...the more I question its sanity..." Created by some programmer who very likely has never seen nor used a real firearm. Just like all these "ballistics calculators" that all the rage. A programmer will tell you it's your rifle if the Real World results don't agree with his program too.
In any case, the twist rate is a standard that's pretty much the same from all manufactures. Only time you get to choose the twist is when buying a barrel. Most factory barrels are the same or very close to it.
The velocity, altitude and temperature have nothing whatever to do with the twist rate. The velocity, altitude and temperature do affect the load though.
"...you need a stable bullet in flight..." Yep. And that can come a lot farther from the muzzle than one usually thinks. For example, it take roughly 300 yards for a .30 cal to stabilize.
 
T O'Heir, before you talk like that you should look into Berger's tools and calculator.

Berger's tools get the Bryan Litz stamp of approval. He's the real deal when it comes to ballistic science, bullet designer, aeronautics engineer, ballistician, and competitive shooter. He is not just some engineer or programmer who has never fired a rifle.

Jimro
 
Created by some programmer who very likely has never seen nor used a real firearm.
Click to expand...
"Firearms" is totally irrelevant.

It needs -- and has -- only an engineer competent in the
physics/dynamics of a rigid rotating body in a fluid.

Been around, and hasn't changed, in more than 300 years.
 
Yup - the idea that bullets stabilize downrange is absurd in the sense that there is no plausible physical mechanism. No one will ever take Brian up on his challenge.
 
There is a caveat to that. There is nonlinear dispersion and then stabilization, but not at the ranges Litz is looking into. Doppler radar tracking out at Yuma for the USMC has produced some very interesting results as bullets pass through the transonic region. Stable going in, unstable during, stable out. Essentially the transonic has some funky wave drag functions that don't show up at faster and slower speeds, so dispersion is essentially predictable before and after, but not during...

But from the muzzle to mach 1.2? Not enough wobble in the bullet flight to significantly change group MOA size at different distances.

Jimro
 
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