Can Someone Explain "rate Of Twist" In A Barrel

[gingerbreadman]

on this particular gun there is a diffeent rate of twist for different calibers.... why is this and what is suited better for different applications, why is the .223 a different rate of twist over the .22-250

http://www.remington.com/firearms/centerfire/700vls.htm

-gbm-

[Sixgun]

Rate of twist is expressed as 1 turn in x amount of inches. Therefore if you see 1:8 that would be one turn in every 8 inches of barrel length.

The reason different calibers have differing rates of twist is to stabilize different weights of bullets. I believe heavy for caliber bullets need faster twist rates than light for caliber bullets, but I'm not 100% sure.

Ray C.

[SDC]

For your specific example (.223 vs. .22-250), the real difference is in the weights and styles of bullets that are expected to be used; a typical .22-250 bullet is going to be a "varmint"-style bullet that you want to have severe expansion/fragmentation when hunting groundhogs, etc., and those bullets tend to be at the lighter end of the spectrum. On the other hand, .223 is used in DCM matches, where you want consistent accuracy at ranges out to 1000 yards, and to have a bullet that will retain it's accuracy at those ranges, you need a heavier bullet; a heavier bullet is going to require a faster twist rate, otherwise the bullet will end up tumbling end over end. In military rifles, they've tried a wide range of twist rates trying to get the best balance between accuracy and wounding capability; originally, M16s were rifled 1:14, and that often wouldn't stabilize even the 55-grain FMJs, so they had poor accuracy at longer ranges, but severe wounding ability. In dense, winter air, the same thing happened, so they next went to 1:12, and after that 1:9. After they adopted the SS109/M855 enhanced-penetration FMJ, they found that they needed to increase the twist rate again to stabilize those heavier bullets at longer ranges, so current rifles are rifled 1:7. If you want a complete run-down on the issue, check out Ezell's "The Black Rifle".

[TBF]

Just my .o2.

The twist rate is related to bullet length, not really bullet weight. Heavier bullets are longer, but so are solid copper and other types of bullets.

Most Mfgs. use a twist rate which works with most common bullets in the particular caliber. If you want to shoot longer than normal bullets, you need a tighter than normal twist.

To find your twist, measure the distance between a complete revolution of your cleaning rod in inches.

Just as info, I have not seen any problems " overstabilizing " bullets in 223 in my 1:8 twist barrel, some say light bullets will not group or even make it to the target ( depending on who you talk to ) . YMMV, I have not shot every load combo known to man.

Travis F.

[Gun Geek]

Below is a copy-n-paste that does a pretty good job of explaining twist rate.

The kinda dismiss overstabilization. For most applications, that is OK, however in true long range shooting, out at 1000yds and beyond, overstabilization can be a problem. It messes up a concept called tracability. If a bullet is over stabilized it can not yaw enough to keep the nose point in the same direction as the travel. If a bullet is on a long distance path (arc) it starts nose up and arcs over until it should hit the ground nose first. Like an arrow. If it is overstabilized, the bullet will stay pointed in the original direction. Problem is that the bullet is not pointed directly into the airflow so it drops velocity quicky and is disturbed more by small wind currents - destroying accuracy.

This website (http://www.nennstiel-ruprecht.de/bullfly/). Gives a very complete analysis of bullet stability. It will make your head hurt, but it has cool diagrams and photos.

To your direct question about this Remington riflie - note that it has a twist of 1 in 12. That tells me they set the rifle up as a long distance varmint gun. With this twist, you would shoot lighter bullets over hot 223 loads (makes sense in a bolt action heavy barrelled gun!). Bolties can run a 223 55gn at about 3500fps, while an AR usually runs them around 2800 - 3100. Notice the 22-250 is a 1:14. You typically shoot 40 - 55gn pills at 4000fps. ARs are typically 1:8 up 1:10. Intended ammo is heavier pills (55gn - 90gn) going a little slower.

[Thanks to Hightechredneck for unravelling my confusion. I was thinking right, but said it wrong. Also, your exactly right about the highpower boys going for very high twist rates.]

http://www.loadammo.com/Topics/July01.htm

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!

[TRG65]

G-man

An excellent book on understanding reloading, and reloading for the 223 in particular is Glen Zediker's book Handloading for Competition. This is a book that doesn't approach handloading from a Benchrest standpoint, but from a Highpower standpoint. I found its advice much more helpful to general loading and loading for hunting than some other books. It is not a load manual, but a how, why and for what reason book.

Gun Geek,

The following two quotes don't make sense to me. I agree with the second part of your post. A 1-12 twist rifle would be set up to stablize light bullets not heavy ones. Highpower competitiors use AR's having the 1-6.5 to 1-8 twists so they can stablize the 69HPBTs to 90 VLDs.

To your direct question about this Remington riflie - note that it has a twist of 1 in 12. That tells me they set the rifle up as a long distance varmint gun. With this twist, you would shoot heavier bullets or hot 223 loads (makes sense in a bolt action heavy barrelled gun!). Bolties can run a 223 55gn at about 3500fps, while an AR usually runs them around 2800 - 3100. ARs are typically 1:8 up 1:10. Intended ammo is lighter pills (45 - 55gn) going a little slower.

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!

[ErikW]

SDC, the M16 1:7 twist was for a long/heavy tracer bullet, not for the 62 gr penetrator, which can be stabilized by much slower twist rates.