Twist rate and Bullet weight

[MattPM0000]

I'm trying to self learn myself. But I have read that a heavier bullet needs less spin in order to stabilize itself. I see that in comparison .223 rounds use a 1:7 twist and .308's can use more of a 1:9. So in this instance it seems to ring true.

The bullet I am looking at using and learning with is the 7mm Rem Mag. The normal suggested twist rate is 1:9. On one of applied ballistics charts for a 175 grain .284 round it suggests a minimum twist rate of 1:9.3 to have the sg equal 1.40. But when I read some of the posts here about Burgers heavier rounds and even in Hornady's reloading manual for their 175gr round, they suggest a twist rate of 1:8. So, with the increase of weight, why does the twist rate increase? Is this because of the decrease in muzzle velocity and requiring compensation?

 

[Gunfighter14e2]

Nose, tail an balance point are what dictate twist rate, not weight. A 240gr 44mag bullet only requires a 1/29 to stabilize it, but a 240gr 30 cal requires a 1/9 or faster. As the bullet gets longer generally it requires more twist to stay stable.

 

[ncr92]

A good read if you want to learn about stability is 'A New Rule for Estimating Rifling Twist An Aid to Choosing Bullets and Rifles' by Don Miller. This paper provides a formula for estimating stability factor. In Miller's formula, stability is a function of projectile mass, diameter and length and rifling twist. Muzzle velocity and atmospheric conditions are fixed for the purposes of this formula but they do have an effect on stability and Miller explains how to adjust calculations to take account of them.

If you take two projectiles that are identical in size and shape but have different masses, the heavier bullet will require less twist to stabilize it. It will have a higher stability factor with the same twist rate. This might be the case if you were comparing a lead core bullet and a copper solid.

If you are comparing bullets of the same construction, higher performance bullets usually require more twist to achieve the same stability factor even if they are heavier. They are made longer and sleeker to achieve a lower drag coefficient and the effect of the increased length outweighs the effect of any extra mass. As Gunfighter alluded to, this is because the extra length moves the center of drag further form the center of gravity.

Miller also provides a formula for required twist. You input your target stability factor (he recommends 2.0) and projectile measurements and it returns a required twist.

 

[XLR308]

I have allways gone with a faster than required twist if there are any questionable stability issues or I plan on using a long heavy for caliber bullet.
The faster twist doesn't have any negative effect unless you plan on using very light for caliber thin jacketed varmint bullets that could be compromised by the faster twist rate.
On the other hand if you go with a slower twist rate you are stuck with the bullet weights it can effectively handle and will have stability issues if you try to push the longer bullets.

 

[Sheldon N]

The longer the bullet, the more twist needed to stabilize.
The slower the bullet speed, the more twist needed to stabilize.
The denser the air (cold temp, sea level) the more twist needed to stabilize.

Try out this online calculator to see what twist rate might work for your application.

http://forum.accurateshooter.com/forums/6br-6br-improved-wildcats.5/

 

[earthquake]

Here's another calculator, GWIW:

http://www.bergerbullets.com/twist-rate-calculator/

 

[sandwarrior]

As long as you are doing the math you might look the guy up who made a science out of it:

https://en.wikipedia.org/wiki/Alfred_George_Greenhill

This is just one article on him. But, as you can see by the formulas provided above it's been a mathematical endeavor by a number of people to figure out exactly what we need and how to get it.

Suffice it to say then, that your initial interpretation is incorrect. That a heavier bullet requires less twist. Those responding to you above me will show it requires more twist.

On thingthat often gets confused is the interpretation of "more" and "less" because "more" is a lower number. A 1-7" twist is more twist than a 1-12" because the bullet rotates in the barrel one complete revolution in 7 inches as opposed to every 12 inches if using the higher number. In fact, in a a 1-7" twist barrel, the bullet will have rotated 1.7 times as fast as a 1-12" twist in the same distance down the barrel.

So, just remember, low twist numbers are FAST or MORE twist. It's the number of inches it takes to rotate one time in the barrel.

 

[MattPM0000]

Thanks everyone for the responses and links. It makes more sense now.