Twist Rate

[tacomaj]

I was looking at the rate of twist on some 7MM rifles.  The .280 was 1/10, 7mm-08 and 7 mags were 1/9.5.

Why the difference since it is the same diam. bullet?  Please  explain the reasoning.  Thanks Tacomaj

[Iowegan]

Because bullets are heavier in the base than the nose, they are required to spin in order to stabilize. An unstable bullet will start to wobble as it travels down range, then it will begin to tumble. Of course, this has a big impact on accuracy, max range, and terminal effects.

Velocity determines the spin rate. Example: a 7mm bullet traveling at 3000 fps spins much faster than the same bullet at 2500 fps. If a barrel has a faster twist rate, it will spin the bullet faster at a lower velocity.

The next factor is the bullet weight. When heavier bullets are used, they can't be driven as fast as lighter bullets in the same chambering. So, a faster twist is required for heavier bullets.

The 7mm-08 optimizes with a 140 gr bullet. The 280 Rem optimizes with a 160 gr bullet and the 7mm Rem Mag optimizes with a 175 gr bullet. These are results from factory tests where accuracy and velocity are best.

So, the 7mm-08 needs a faster twist to stabilize a slower bullet. The 280 Rem needs it for a heavier bullet. The 7 Mag is way higher in velocity and doesn't need as fast of twist even though is likes the heavier bullets.

[leverfan]

Twist rates also vary by manufacturer, with each company deciding what they think will work best with the bullet weights that the rifle is likely to be paired with.  RPMs are really what stabilize the bullet, so that's why high-velocity cartridges can get by with slower rates of twist.  The higher velocity produces higher RPMs, and RPM degrades at a slower rate than velocity as the bullet travels downrange, so everything works out.  Heavier bullets need a higher RPM because they are longer than light weight bullets, therefore they are harder to stabilize in flight.  Barnes bullets, polymer tipped bullets, and other long-for-weight designs may also require a faster twist or higher velocity to reach a stable RPM range.  Twist can effect pressure as well, so it's a balancing act for the barrel maker, but it's usually better to err on the side of "too fast" of a twist, rather than too slow.  You can't really over-stabilize a short bullet, but you can sure tumble a long bullet with a slow rate of twist.  Sierra puts a table of rates of twist for rifles and handguns in the back of their reloading manual, and it lists which gun makers use which twists for a given round.  It's obvious that the bean counters at some gun plants have decided that all 7mm barrels should have the same rate of twist, saving on production costs, and the final chambering is not considered.  There's more that could be said about this topic, but I've probably put everyone to sleep by now.

[Iowegan]

Leverfan, You got a good start on twist rate. Actually, it's the diameter of the bullet that most affects stability. A large diameter bullet will stabilize much easier than a small diameter bullet. The length has very little to do with stability, nor does weight, assuming the same velocity.

When a heavier bullet is used, it can't be driven as fast as a lighter one in the same cartridge else over pressure will result. Therefore, it will spin slower. To achieve stability, the twist rate must be faster to accommodate the slower velocity.

Unfortunately, we can't change our rifle's twist rate without changing the barrel. Rifle and ammo manufacturers go through exhaustive tests to find the "right" twist rate for a given cartridge.  When a twist rate is determined, it will work with a range of velocities.  If the velocity is too slow, the bullet won't stabilize. If the velocity is too fast, the bullets start skipping over the lands and grooves and they act more like a file than rifling.  Evidence of too high a velocity is excessive jacket residue in the barrel.

This boils down to a range of bullet weights that will produce satisfactory results. As stated before, the reason it is bullet weight dependent is because the velocity slows as the bullet weight increases, not because heavier bullets are more unstable.

[leverfan]

Leverfan, You got a good start on twist rate. Actually, it's the diameter of the bullet that most affects stability. A large diameter bullet will stabilize much easier than a small diameter bullet. The length has very little to do with stability, nor does weight, assuming the same velocity.

Iowegan, you have a good start on twist rate, too, but I'm afraid that you may actually be the one that's confused.  Don't take my word for it, though-

"The general rule of thumb is that a longer bullet in relation to its diameter will require a faster rate of twist than a shorter bullet.  Also, a more pointed bullet will need a faster twist than a blunt bullet (because it's longer).  Finally, velocity can affect the rate of twist needed to stabilize a bullet.  A given bullet will stabilize with a slower twist rate if the velocity is higher." Lyman 48th ed., page 104, emphasis added.

"Everything else being equal, long bullets need  a faster rate of twist than short bullets.....Tom Bricker of Manheim, Pennsylvania, makes his 7mm woodland bullets at 190 grains.  He said with the pointed flat base bullet in that weight the standard 1:9 rifling in a 7mm Remington Magnum will stabilize the bullet.  Bricker makes a 190-grain pointed boattail in 7mm which he said needs a faster twist to shoot accurately."  ABC's of Reloading, 6th ed., pages259-260, emphasis added, and it contains a clear example of bullets of the same weight and diameter, but different length, requiring different rates of twist.

"The 244, as it was introduced, had a twist rate of 1 in 12", which left it unable to stabilize long 100-grain Spire Point bullets and thereby reduced the versatility of the round.  (However, Hornady offers a 100-grain Round Nose bullet for the rifles with a 1 in 12" twist.)"  Hornady Handbook, 5th ed., page 175, emphasis added, and another example of how it's the length, not the exact weight, that matters for twist.

If you reload for an old 1:38" twist Marlin 444, or if you shoot 160 grain 6.5mm bullets in a 1:9" twist gun, or if you wonder why flat points are more accurate than spire points , you quickly become a student of rates of twist and how it affects the bullets' flights and terminal ballistics.  It's all about bullet length and rpm, and the only differences that bullet weight makes are:  Is the heavier bullet longer?  Does the greater weight, and reduced velocity, drop rpm's too low to stabilize that length of bullet?  That's why conventional cup-and-core bullets are easier to stabilize than the long-for-weight Barnes copper bullets, various polymer tipped bullets, etc., even when bullet weights are identical.  At the extreme end, an all-plastic, 150 grain .308 bullet would need a far faster twist than the 1:12" that works so well in the 308 Winchester when it's loaded with conventional bullets.  Why?  That plastic bullet would be several inches long, and it would be a beast to stabilize it.  Diameter isn't of any great importance, here.  If you still don't believe me, feel free to contact Sierra's Ballistic Services dept. at 1-800-223-8799, or e-mail them at sierra@sierrabullets.com .  You'll get the same answer there, too.  

This post isn't intended as a flame, I just want to help clear up any confusion, and make sure new shooters are getting the correct info. :-)

[Iowegan]

Leverfan, Your post has both truth and contradiction.  I agree that RPM's is where stability comes from. There are two ways to change the RPM's, twist rate and velocity. The higher the velocity, the faster the spin, and the faster the twist rate, the faster the spin. I think we agree on this.

If you look at any twist rate chart, you'll see the larger the bullet diameter, the slower the twist rate. These large heavy bullets are also traveling much slower than those of smaller diameter and weight so their actual RPM's are much slower.  Note your reference to the 444 Marlin with a twist of 1:38.

Long sharp pointed bullets don't always require higher RPM for stability. Mostly it's a balance issue. All rifle bullets are lighter in the nose than in the base and are naturally unstable. The heaviest part wants to lead and in order to keep the lighter end in front, the bullet must be spun very fast.

Lets take a 38 Special, dual-end wad cutter as an example. The bullet is nothing more than a lead cylinder. The balance point would be dead center on its horizontal axis. Labs refer to the balance point as a front-to-back ratio. This perfect cylinder would have a front-to-back ratio of 1:1 and would require very little spin to keep it stable. Now lets sharpen the point a little and create a round nose bullet, keeping the weight and diameter the same. Our front-to-back ratio changes to 1:.9, indicating the base is slightly heavier than the nose. Now we must spin it a little faster to get it to stabilize. Now lets sharpen the point even more making a spire point. The bullet still weighs the same and is still the same diameter. The front-to-back ratio will now be 1:.8 and you'll notice the bullet will be considerably longer. The balance point has moved more toward the base. We haven't changed the weight, so we can still use the same powder charge and get about the same velocity, thus the same RPMs. Notice I said "about the same velocity". Actually, velocity will increase slightly with the same powder charge because the bearing surface of the spire point would have less bore friction, thus slightly faster velocity.

Lets look at rifle bullets. A hollow point or polymer tip bullet would have a front-to-back ratio down around 1:.6. That means the base is quite a bit heavier than the nose.  This bullet is naturally unstable and will want to "swap ends" in flight. The solution is to make it spin faster, either with higher velocity or a faster twist rate. Another thing that helps is to lighten the base by forming a boat tail. The boat tail will move the balance point back closer to center and it will be much more stable. Still another trick that bullet manufacturers do is to make a good share of the base with gilding metal (copper alloy). It is much lighter than lead so the balance will move forward and the f-t-b ratio will get closer to 1:1. Nose heavy bullets are also just as unstable. The closer to a 1:1 f-t-b ratio, the better it will stabilize. So your statement about bullet length really depends on how the bullet was constructed and it's f-t-b ratio, not length alone.

Generally, most rifle bullets are in the 1:.7 f-t-b ratio or higher. Anything less will be difficult to stabilize. As the length increases, so does the weight and bearing surface. More weight means a slower velocity and less RPMs, thus requiring a faster barrel twist.

Going back to diameter. It's a basic law of physics known as the gyro effect. The larger the diameter, the less RPMs it takes for stability.  That's why "books" say heavier bullets need a faster twist rate. What they really mean is: within the same chamber, heavier bullets need a faster twist rate. Taken out of context, this "heavier needs faster twist" statement is dead wrong.

In reference to your statement about bullets retaining their spin rate. Again this is a basic physics thing dealing with friction. The more aerodynamic (BC) the bullet the more spin it retains. The less aerodynamic bullets are influenced more by the air friction and slow the spin at a faster rate.

If you want to compute your spin at the muzzle: RPMs=12/TW*MV*60 Example: A 22-250, 55 gr bullet with a 1:14 twist rate @ 3600 fps. Convert inches to feet by dividing 12/14. or .857 spins per foot.  Now multiply by the MV or .857*3600=3086RPS. Multiply by 60 to get RPM or 3086*60=185,160 RPM.  The 444 Marlin with a 300 gr bullet and a 1:38 twist @ 2100 fps: 12/38=.316  .316*2100=663.6  663.6*60=39,816 RPM. You can see the little .224 diameter bullet has to spin about 4.6 times faster than the .429 bullet to stabilize. This example should prove that heavier bullets or slower velocities alone, do not determine stability.

I may be an old fart but I did retain a little from physics class and several years as a technician in a ballistics lab helped too. Your statements weren't wrong, they were just taken out of context, which really changes the meaning. Your references were righteous but again, you need to look at the whole picture, not just a statement.

In conclusion, bullet diameter is the single biggest factor for stability. The other factors are all based on your barrel’s twist rate and how fast you can drive the bullet, ie: weight, length, shape, and of course F-T-B ratio. These concepts are common knowledge with bullet design engineers but probably will not show up in any reloading manual.

[Rojelio]

Does anyone still use the Greenhill formula to determine the required twist to stabalize a certain bullet?  (T)twist=150xdiameter squared divided by the length. My 129gr. SSTs (.264) require a 7.94 twist. diameter squared =.069696x150=10.4544 divided by length (1.316)=7.9440729. Tell me if I'm wrong. Rojelio

[Iowegan]

I've never seen that formula. The one I used to use did not have "length" but did have velocity. I can't see where you could possibly calculate the twist rate without knowing the velocity.  The length of the bullet has very little to do with the twist rate. I must say, your math does agree with the standard twist rate chart though, so it may be valid. Note: from 9mm and larger, the listed calibers are for pistol. Rifle twist rates are much slower due to increased velocity.

Caliber   Twist   
.172   10"   For all bullets

.22 RF   14"*   Twist for pistol barrels
   16"   Standard twist for rifle barrels
   17"*   Special twist for rifle barrels

.224 CF   9"   For bullets heavier than 63 gr.
   12"   For bullets up to 63 gr.
   14"   For bullets up to 55 gr.
   15"*   For bullets up to 55 gr. driven 4,100 fps or more
   16"*   For bullets up to 55 gr. driven 4,300 fps or more

6mm/.243   8"   Special for VLD bullets
   10"   For bullets up to 120 gr.
   12"   For bullets up to 85 gr.
   13"*   For bullets up to 75 gr.
   14"*   For bullets up to 70 gr.
   15"*   Special for bullets up to 70 gr.

.257   9"   For bullets heavier than 100 gr.
   10"   For bullets up to 105 gr.
   12"   For bullets up to 90 gr.
   13"*   For bullets up to 80 gr.
   14"*   For bullets up to 70 gr.

6.5mm/.264 8"   For bullets heavier than 120 gr.
     9"   For bullets up to 120 gr.

.270   10"   For all bullets

7mm/.284   9"   For bullets heavier than 140 gr.
   11"   For bullets up to 140 gr.

.307   13"*   Special size and twist

.308   8"   For bullets heavier than 220 gr.
   10"   For bullets up to 220 gr.
   12"   For bullets up to 170 gr.
   14"*   For bullets up to 168 gr.
   15"*   For bullets up to 150 gr.

7.65mm/.311 10"   For all bullets

.338   10"   For all bullets

9mm/.355   14"   For low-velocity wadcutters
   16"   For all other bullets

.38/.357   14"   For low-velocity wadcutters
   18"   For all other bullets

.358   14"   For all bullets

.375   12"   For all bullets

10mm/.400 16"   For all bullets

.411   14"   For all bullets

.416   14"   For all bullets

.44   20"   For all bullets

.451   16"   For all bullets

.458   14"   For all bullets

[leverfan]

Rojelio-

You're not wrong, and the Greenhill formula is still in common use.  It was used to determine the proper rate of twist for the old Marlin 444, as it was originally loaded with 240 grain (relatively short) soft points.  In this case, twist is calculated:  (.429x.429x150) divided by the length of a .429 diameter 240 grain softpoint (.72), which gives an answer of 38.341875, or 38" twist.  This is actually a slightly modified version of the Greenhill formula, using his constant (150), but the results are the same.  They're just harder to calculate if you do it the way that Sir Alfred did, because his method required switching between calibers and inches.  For instance, the bullet in my example is 1.68 calibers long, so the constant of 150 is divided by 1.68, giving a twist rate of one turn in 89.29 calibers.  When you convert calibers back into inches by multiplying by .429, you get 38.3.  

Rojelio, your method is obviously faster, and is the preferred method these days.  Many people, when ordering a custom gun that's designed to shoot a single, favorite bullet, will use the Greenhill formula to determine the exact twist that will give the best performance with the chosen bullet.  Factory rifles tend to have a more "generic" twist that will perform well with a wide variety of bullets.

Iowegan-

We'll have to agree to disagree, as I feel that the diameter of the bullet only matters as part of a relation between length and diameter.  Here's a little something from the rifle rate of twist tables in the back of the latest Sierra manual, which shows how rifle bullets of radically different diameters and velocities have the same rate of twist in different rifles (by the way, your little table is wrong right off the bat when you say that .172" bores have the same twist for all bullets, Remington has a 9" twist).  All of the following have 1:14 twists:

22 Hornet (Kimber, Savage, Cooper, Ruger)
6mm PPC (Sako, Ruger)
257 Weatherby (Mannlicher-Schoenauer)
35 Remington (T/C)
375 H&H (old Brownings)
404 Dakota (Dakota, of course)
416 Remington (most makers besides Ruger)
45-70 (T/C)

Other makers use different twists for the same exact cartridges, often because they envision the shooter using different length bullets.  I don't mean to suggest that a 1" long .224" diameter bullet needs the same twist as a 1" long .458" bullet, because it's not a constant relation.  The 1" long .224" bullet will need a faster twist to stabilize because of it's long in proportion to its diameter.  Perhaps that's the source of the confusion here, but I hesitate to take advice on twist from a poster that's never heard of the Greenhill formula, perhaps the most widely used method of determining twist rates for modern arms for decades.

[leverfan]

Because bullets are heavier in the base than the nose, they are required to spin in order to stabilize. An unstable bullet will start to wobble as it travels down range, then it will begin to tumble.

I just thought that I may as well revisit this opening line from your first post on the subject, because it's flat wrong.  Even spherical or cylindrical bullets need to spin in order to stabilize and provide acceptable accuracy, so I'm not sure where you got, "Because bullets are heavier in the base than the nose, they are required to spin in order to stabilize."  Through trial and error, gunsmiths of old determined that round balls were most accurate when they were fired with a spin of one turn in about 6 feet.  This comes within inches of what the Greenhill formula will suggest, by the way, but of course they didn't have access to it.  Anyway, all bullets are required to spin in order to stabilize, even roundballs and wadcutters.

[Iowegan]

Leverfan, If you look at a round ball, it has a perfect front-to-back ratio and yes, if you spin it a little, it will be extremely stable. I never said bullets didn't need to spin, I said bullets that were base heavy need to spin and should have added "considerably faster".

Sorry for the bad twist rate chart. I should have looked at it a little closer after I downloaded it and included it in the post. A piece of junk really.

For you "non-believers" in the balance issue. Show me a bullet where the balance point is dead center in the horizontal plane and I'll show you a bullet that is very stable. Likewise, show me a bullet that is tail heavy and I'll show you a bullet that is NOT as stable. The only bullets I know of that are nose heavy are hollowbase wad cutters. Driven at slow velocities, they are very stable at "pistol shooting distances". I've never tested them at long range because there's no application for it. I suspect the HBWC's, being nose heavy, will get very unstable if driven faster.

As far as the comment on the Greenhill formula, I'm not doubting it's validity because the calculations came out very close to the charts. The formulas we used in the lab were considerably different and were very accurate. That was almost 40 years ago and I don't remember the equation, hell, I can't remember my bosses' name back then. But I will stick by my guns on the diameter and gyro theory.

No matter what formula you use, the real proof is taking the rifle out and shooting some long range targets. At short distances, say 100 yds, most bullets will put a nice round hole in the target.  Try it again at 300 yds. Some bullets will start to wobble and produce oval holes, the first indication of instability. And that might be good enough for most hunting ammo. Now go out to 500 yds. Here's where bullet stability really shows up. I've seen a good many bullets produce keyholes at this range, and we haven't even got to the extreme ranges some long distance shooters like.

When I worked at the lab, we tested ammo and rifles from all US makers and several foreign makers. It was always a challenge to see how different bullets performed in different guns. The "keyhole" factor was well documented for all the factory ammo we tested. Even in popular calibers such as a 30-'06, it was not unusual to see keyholes as soon as 200 yds. I wish I still had access to that data.

Going back to bullet spin vs velocity. The labs say the spin (RPMs) decays much faster than the velocity. This is evident when a bullet makes a nice round hole at 100 yds, and keyholes at 300 yds. If the RPM's didn't decay, a bullet that is stable at 100 yds would also be stable at 1000 yds, and we all know that doesn't always happen.

Sorry if I offended anyone, just trying to project my side of the issue.

[leverfan]

Iowegan-

This is probably a case of a conversation that would have taken all of 5 minutes over beers before we each came away happy, having learned from each other.  I like to hash things out, but I'm aware that I come on a bit strong, especially over the internet (where folks can't see that I'm saying things with a smile).  Anyway, I think we're both actually circling the same point, just coming from different directions.  Thank you for being a gentleman during the discussion, and I'm sorry if I came across as anything less than that.

[Huk]

H'lo,
On the subject of twist rates.
The standard twist for a .22 rimfire is 1-16" yet some of the newer Ruger 10/22's are comming out with 1-14". And Remington shows their benchrest target .22 with a 1-14" twist as standard and the 1-16" as optional.
Anyone know why?
I have a 10/22 with the 1-14" twist and get excellent accuracy with it.

Just wondern'


Huk