Comments on: Ballistics Tip: Understanding Bullet Stability (Twist Rate and MV)

By: Justin rockwell

Justin rockwell — Tue, 25 Jun 2019 23:03:09 +0000

So stability factor changes as velocity decreases as the bullet travels down range? You mention the 1.5 sf but is that at the muzzle or is that entering the transonic zone?

By: Editor

Editor — Tue, 08 Sep 2015 06:22:45 +0000

As Bryan explained, RPM is a meaningful measure of bullet rotation. We used this term because the acronym is familiar to most readers. There is a simple formula to calculate Bullet RPM with any MV and twist rate:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

If you imagine this is a Dasher bullet launched at 3000 fps, then it spins at 180K rpm in a 1:12″ twist barrel vs. 270K rpm in a 1:8″ twist barrel. That should be understandable.

By: Bryan Litz

Bryan Litz — Mon, 07 Sep 2015 00:09:53 +0000

BC is not a factor in determining stability (SG). The reason for inputting BC is so you can see what the BC degrades to if the bullet is not fully stabilized. If your parameters result in full stability (SG>1.5) then the BC you input will be unchanged. But if the SG is below 1.5, the BC begins to be affected and the Berger Stability calculator shows you how much.

Stability calculations and the discussions surrounding them can get very complicated very fast. Typically, you want to consider inches per turn (or turns per some distance) because that measure is most closely related to stability (although it’s not directly proportional due to aerodynamics at different Mach numbers…) However, if you talk about RPM’s at a certain fps (both time based rates) then you can do some converstions and get to Revs per foot, and revs per inch/inches per rev. They’re just different ways to do the math; one is more direct and intuitive but the other (RPM’s and fps) isn’t ‘wrong’, just less direct.

For those interested in an in-depth exploration of stability and its effects on BC in both supersonic and transonic flight, check out: Modern Advancements in Long Range Shooting-Volume 1. This book presents all the live fire data that supports the link between stability and BC.

-Bryan

By: mikecr

mikecr — Sun, 06 Sep 2015 23:05:06 +0000

This article implies way more(and different) than Bryan said.
Forget RPMs..
Stability does not come from turns per time, it comes from displacement per turn minus other moments to overcome.
Displacement being air density/drag.

Drag is tied to velocity, but this has nothing to do with RPMs. So any RPM correlations to stability will fail tests.

By: ELR Researcher

ELR Researcher — Sun, 06 Sep 2015 21:00:34 +0000

Hm…Seems whether you select G1 or G7 the SG come out the same.

I input a set of data including .970 for the BC and got a result. I then noted that the button was set for G7 when the .970 BC value was a G1. I cleared the screen back to the base .308 projectile, re-input the values to match the prior setup but with the BC value set for a G1 and reran the number. Same SG.

Really?

By: Mr Bill

Mr Bill — Sun, 06 Sep 2015 15:26:11 +0000

NC=BC Goony finger at work . . .

By: Mr Bill

Mr Bill — Sun, 06 Sep 2015 15:25:10 +0000

I see the calculations start with BC. Since NC varies with velocity wouldn’t it be better to ask for the drag factor? I could be way off on that one but isn’t a constant more desirable than a variable in most cases?