I know a lot of you probably have this information but it seems 'Twist Rates' come up often on the forum. This seems to be a simple explanation of the Greenhill formula which might help some - I found it to be an interesting read... I am sure it is not as simple as the article suggests but it certainly gives you some guidelines....
Quote:
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!
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Yes Twist is Important, just ask Chubby Checker,LOL. All kidding aside a well explained article on Twist, I just learned something new and now I have one more thing to experiment with, darn!
(BP)
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Sabotloader
what would you consider to be the best twist rate for a modern Inline shooting Sabots, also what barrel lenght would you prefer and why?
(BP)
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Those who hammer their Guns into plows will plow for those who do not. Thomas Jefferson
I am a total novice at this stuff - Lee is far closer to being a pro than I.... but of all the rifles that I have - I really think the most versitile twist rate I have is in the White U-Mag - which is a 1:24. I can shoot huge long conicals and I can shoot equally well .4/200 grain Hornady XTP's.
I do prefer the 26" barrel for the type of hunting I do. I think I get a more complete powder burn in the longer barrel and I also believe it improves accuracy. Although there are times hunting the brush that i really like my 24" Remington 700ml.
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Keep on Shooting Muzzleloaders they are a Blast
According to the formula, the optimum twist for .40/200 XTPs is somewhere around 1:35, and for .410/210 GDs is 1:37.
My much loved .452/300 GDs should like 1:38.
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My wife says I'm totally nuts, but I think I'm Semisane.
Things I've Learned: (1) It's not possible to please everyone, but quite easy to piss everyone off. (2) If you love animals as I do, then you're not a vegetarian. (3) There's no need to act stupid, even if you're very good at it. (4) If you eat right and exercise, don't smoke or drink, you're going to die anyway.
Just remember this is am approximation... and then remember the 'over stabilization' statement... so your 1/28's are close and if your bullets are round and the weight is distrubuted evenly you should not get wobble from the faster twist rate as long as it does not get to fast....
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Good point. By the time I computed the figures I forgot all about the "over stabilization" part. Yep, oldtimers disease.
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My wife says I'm totally nuts, but I think I'm Semisane.
Things I've Learned: (1) It's not possible to please everyone, but quite easy to piss everyone off. (2) If you love animals as I do, then you're not a vegetarian. (3) There's no need to act stupid, even if you're very good at it. (4) If you eat right and exercise, don't smoke or drink, you're going to die anyway.
Can you run the formula backwards, knowing the twist and the caliber, to derive the optimum bullet length/weight?
Remember weight does not figure into the formula... Weight is considered in the caliber and length. The problem there is Barnes and Lehigh bullets that do not have lead so they use and extended length to make up the weight.
TR=150*Diameter squared/Length
So to detirmine L (length) use this formula 'D' being the diameter of the bullet you want to use squared
L= 150*D2/TR
I think that works....
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Keep on Shooting Muzzleloaders they are a Blast
Bullet Length/Diameter - Twist rate
