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Old November 4, 2004, 07:06 PM   #1
Drjones
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Please explain barrel twist ratios to me.

What is barrel twist?

How does it affect accuraccy/ballistics?

What does 1:7, 1:12, 1:14 mean?


Any info is appreciated.


Thanks!
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Old November 4, 2004, 08:10 PM   #2
esheato
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The ratios (1:7, 1:9) indicate one complete turn of the bullet in so many inches of barrel. Soo...a 1:9 twist indicates one complete turn in 9 inches of length.

Barrels also have left or right hand twists. Look into the barrel of an unloaded weapon and you can see which direction the barrel twists. To determine your twist rate, fit a tight fitting patch on a jag and put it on a cleaning rod. Insert it into your barrel, mark the cleaning rod, push the rod into the barrel until it makes a complete revolution, and mark the barrel again. Remove the rod and measure the distance between the marks, and that's your twist rate.

Different bullets require different twist rates to stabilize the bullet properly.

For example,

Berger Bullets lists all their products and the recommended twist rates for optimum accuracy. If the bullet isn't stabilized properly, poor accuracy is a result.

There's more to it than that, but that's basically how it works.

Ed
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Old November 4, 2004, 08:13 PM   #3
DAVID NANCARROW
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The ratio, such as 1:12, means the bullet will make one complete revolution in 12 inches. Bullet spin is like like a perfectly thrown spiral in football. Twist rate is dependent on the diameter and length of the bullet. It provides the stability the bullet needs for long, accurate distance.
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Old November 4, 2004, 08:22 PM   #4
esheato
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Quote:
Bullet spin is like like a perfectly thrown spiral in football.
That's what I was trying to say! LOL

Ed
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Old November 4, 2004, 08:37 PM   #5
Drjones
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Thanks.

How do the various twist rates affect velocity, accuracy, etc?

In general, what does a higher rate of twist translate to in real-world performance?

Same for lower rate of twist.


Thanks
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Old November 4, 2004, 09:02 PM   #6
JohnKSa
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The longer the bullet the faster the twist must be to stabilize the bullet.

At one extreme, muzzle loaders designed for ball ammo may have twists in the 1:30 to 1:40 range.

At the other extreme, 80 grain .223 bullets need twists of 1:7 to make them stable.

If the twist is too slow, the projectile will wobble or even tumble--both of which will hurt accuracy. If it is too fast the barrel will wear faster and if it is REALLY too fast the projectile can explode from centrifugal force.

If a bullet is stabilized by a given twist, it's stable. You don't get an improvement on an already stabilized bullet by trying to spin it faster.

If a bullet is marginally stable then a faster twist will noticeably improve accuracy.

A faster muzzle velocity will increase the spin rate of a bullet in a given twist barrel. However, this effect is not usually useful since it's rare to be able to increase the muzzle velocity enough to increase the spin rate significantly.
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Old November 4, 2004, 09:38 PM   #7
DAVID NANCARROW
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The ideal twist rate is a ratio where the bullet will "go to sleep" in a fairly short distance from the muzzle. When the bullet first exits the muzzle, it is being pushed, tail first by the expanding gasses. This typically imaparts a slight wobble to the projectile. If the bullet has a concentric jacket-meaning the jacket is the same thickness all the way around the circumference, and the spin is correct, the bullet will stabilize and fly true.

As mentioned by JohnKSa, increasing velocity will sometimes stabilize a particular bullet in marginal twist barrel application. It cannot increase the rate of twist-that is fixed by the rifling, but it does increase the bullets rpm, and will stabilize a bullet if it can be safely driven fast enough. A typical twist rate for a 308 Winchester is 1:12, but some shooters will go with a 1:13 and drive the bullet faster in order to get stabilization, plus the velocity they want in 1000 meter matches. The idea is to have the bullet supersonic for the entire flight. That gets into another kettle of fish, as the shock wave moves from the nose to the bullets tail.

How much a twist rate affects pressure would be a good study and I would like to hear at least some theoretical on this.

I have heard from a lot of people that the heavier the bullet, the tighter the twist must be. I'd like to know if increasing the weight of the bullet without adding length, such as a tungsten insert, if this is still the case.
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Old November 6, 2004, 02:31 AM   #8
JohnKSa
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Length is the key. If you can make it heavy but not longer then it will still stabilize.

That's why you get into apparent contradictions with the .223 round. If you look around, you'll find some bullet makers saying that a 62 grain round can be stabilized with a 1:12 twist. But if you go over the the assault forums you will see them recommending a 1:9 twist for the 62 grain rounds.

The military 62 grain bullets have a steel core which makes them long for their weight. The extra length needs a faster twist for stabilization while the more conventional bullets with copper over lead are shorter for a given weight and can be stabilized by a relatively slow twist.

Of course, the heavier the round the slower the muzzle velocity--that contributes a bit to needing a faster twist. The assault weapon folks are typically using shorter barrels too.
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Old November 6, 2004, 08:19 PM   #9
jefnvk
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Is there any sort of formula that you can plug variables into, to figure out the optimal rate?
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Old November 6, 2004, 09:34 PM   #10
JohnKSa
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Yes...

Quote:
Greenhill’s Formula - A formula that relates bullet weight and length to rifling twist. Having two of them, the third can be calculated. One must just keep in mind that Greenhill's formula assumes cylindrical, pure lead bullets, and doesn't work very as well for small calibres as it does for large bores. L = bullet length in inches D = bullet diameter in inches [150/(L/D)]xD = twist in inches.
Here's a more thorough treatment of the topic.
http://www.loadammo.com/Topics/July01.htm
Topic of the Month: July 2001
Twist Rate

Bullet stability depends primarily on gyroscopic forces, the spin around the longitudinal axis of the bullet imparted by the twist of the rifling. Once the spinning bullet is pointed in the direction the shooter wants, it tends to travel in a straight line until it is influenced by outside forces such as gravity, wind and impact with the target.

Rifling is the spiral or helix grooves inside the barrel of a rifle or handgun. These grooves were invented a long time ago, perhaps as early as the 14th century. However, the smooth bore, using the round ball, was the choice of weapons for warfare even through the American Revolutionary war. The smooth bore musket could be loaded faster than the rifle and didn’t foul, as bad, with the combustion products of black powder.

The rifling grooves helix is expressed in a twist rate or number of complete revolutions the grooves make in one inch of barrel length. A 1in10 or 1:10 would be one complete turn in 10 inches of barrel length.

How important is twist rate? David Tubb, a winner of several NRA High Power Rifle Championships, was using a .243 rifle with a 1 in 8.5 twist barrel. He wasn’t able to get consistent accuracy until he changed to a rifle barrel with a 1 in 8 twist. The ½" twist change made all the difference between winning or losing the match.

A term we often hear is "overstabilization" of the bullet. This doesn’t happen. Either a bullet is stable or it isn’t. Too little twist will not stabilize the bullet, while too much twist, with a couple of exceptions, does little harm. Faster than optimum twists tend to exaggerate errors in bullet concentricity and may cause wobble. The faster twist also causes the bullet to spin at higher rpm, which can cause bullet blowup or disintegration because of the high centrifugal forces generated. For example, the .220 Swift, at 4,000 fps., spins the 50-grain bullet at 240,000 rpm.

One of the first persons to try to develop a formula for calculating the correct rate of twist for firearms, was George Greenhill, a mathematics lecturer at Emanuel College in Cambridge, England. His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:

Twist = 150 X D2/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The twist formulas used in the Load From a Disk program, featured at this web site, uses a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 fps.

The Greenhill formula is simple and easy to apply and gives a useful approximation to the desired twist. The Greenhill formula was based on a bullet with a specific gravity of 10.9, which is about right for the jacketed lead core bullet. Notice that bullet weight does not directly enter into the equation. For a given caliber, the heavier the bullet the longer the bullet will be. So bullet weight affects bullet length and bullet length is used in the formula.

To measure the twist of a barrel, use a cleaning rod and a tight patch. Start the patch down the barrel and mark the rod at the muzzle. Push in the rod slowly until it has made one revolution, and then make a second mark on the rod at the muzzle. The distance between marks is the twist of your barrel.

To see how this works out, assume you bought a .222 Remington rifle and you measured the twist rate as described above. The twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain Speer SPS and the 50-grain Hornady SX. The Speer bullet measures .812 inches in length and the Hornady measures .520 inches. Using the formula above we calculate the following twist rate:

Speer 70-grain: 1 in 9
Hornady 50-grain: 1 in 14

These calculations show that the 50-grain bullet will be stabilized, but the 70-grain won’t. Sure enough, when you try these bullets out, the 50-grain shoots ¾ MOA while the 70-grain won’t group on the paper at 50 yards. Twist is important!

Watch our web site for the next topic of interest "What is Recoil and how is it Calculated." Until then, shoot safely and know where you bullets are going.

Sincerely,
The Ballistician
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