[Eb1]

look here for example: http://www.uslink.com/~tom1/twistrate.htm and here:
http://www.loadammo.com/Topics/July01.htm


Where in this equation do you see weight? Why does everyone say that bullets are to heavy? I know, I know. Longer bullets are heavier. Right?

It is very confusing, and I think that most everyone would benefit by doing a little research if they are going to look for accuracy, or give others information as fact on the internet.

Weight does not directly enter into a twist rate.......:banghead:


Think of your rifle as a car when you were a teenager. You learned the ins and outs of the automobile so that you could get the most out of it. You understood the machine. Why not do this with your other machines.

[Mal H]

Perhaps this should be in the Rifle forum? At any rate, it appears to be the answer to an unasked question, or at least an answer in search of a thread with the question.

What prompted the mild rant, Eb1? Obviously you are addressing the age old, "that twist rate won't work with that weight of bullet", but why do it in this manner?

[dmazur]

There are quite a few "empirical" formulas running around that are used to design various things. Manning's equation comes to mind, for flow in an open channel.

Here's a cite from Wikipedia -

The Manning formula, known also as the Gauckler-Manning formula, or Gauckler-Manning-Strickler formula in Europe, is an empirical formula for open channel flow, or free-surface flow driven by gravity. It was first presented by the French engineer Philippe Gauckler[1] and later re-developed by the Irish engineer Robert Manning in 1890. For more than a hundred years, this formula lacked a theoretical derivation. Recently this formula was derived theoretically[2] using the phenomenological theory of turbulence.

Note that it was used, but lacked a theoretical derivation, for over 100 years.

Back to bullet weight -

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.

This seems pretty simple to me. Rather than a full-out derivation using physics equations, someone has found a formula which provides an approximation for twist, using bullet length. Bullet weight affects bullet length and that is used in the formula.

One should not read this as "bullet weight does not matter" as that is absolutely incorrect. "Based on a bullet with a specific gravity of 10.9" says weight is involved. The simplification is that the relationship to weight can be referenced more simply by simply using the bullet's length and diameter, and assuming a certain specific gravity.

I'm all for simplified empirical equations that produce repeatable results and that can be used as tools to solve a problem. We're not trying to impress a calculus professor here... :)

[Sport45]

We use weight because everyone pretty much knows what their particular bullet weighs. I, for one, could not tell you the length of any bullet I shoot.

As you said, longer bullets are heavier. Unless you are talking about tracers, steel core, or solid copper bullets that is a generally true statement.

[Eb1]

Mal H,

My appologies for coming across as a rant. Really it was just to stress the fact that heavier doesn't mean it will not work.

Lately I have been asking questions about BC because it is new to me, but I am also reading posters tell users that their barrel twists limits them to a certain bullet weight range as factual answers, and this is a false statement to the users who are in search of an answer. This troubles me a little. It is not fair to the shooting community to be lead down the wrong road in which they are seeking optimum performance and ballistics from their rifles. It just seems wrong to tell things as facts when they are not, but there is a way to formulate the answer to be fact. I even see in the example equations I posted that there might be some flaws in those equations, but for the most part they are close to being exact.

To make a blanket statement like a 1:9 twist will only shoot xx grain to xx grain is quite disturbing. Yes, it has limitations, and they can be determined by doing the math. What frustrates me is that this is not explained, and general opinion is that because you have a certain twist you gun will not shoot a heavy bullet. Which is false. It will not shoot long bullets.

I tried the 75 grain A-MAX. It is 1.13" or thereabout, and it would not shoot in my 1:9 twist rifle. I tried the 75 grain HPBT from Hornady. It's length is 0.985 thereabouts. It shoot pretty good for a first attempt, and I am thinking it will shoot better with a slower powder along with some testing. There are other factors as well. Bullet shape and such, but it is however a 75 grain bullet after all. Is it not?

My rifle shoots the 69 grain SMK like a laser at 2850 thereabout average FPS. I mean like a laser. Wait... Someone said my 1:9 twist is only good for 60 grain. No, it will only shoot 55 grain. No, 62 grain is the best! And lord knows it will not shoot a 75 grain bullet. Why must a poster have to listen to this mess time and time again? Sure. A particular rifle will shoot a particular bullet better than the other. And that bullet might weigh 52 grains or 69 grains, but to tell someone that it will not shoot a bullet of a certain weight is just not true unless the math says it is.

I wasn't trying to make a rant on thehighroad, but I was trying to get the attention of shooters who listen to the blanket statements without knowing why and how the conclusion of what a gun's barrel twist will allow them to shoot.

I guess I am just the type of person who wants to know why something will or will not work. I ask questions, but I research the answers I get. I compare them to my answers, and go with that.

I think this thread belongs in Handloading. Because it deals with handloaders asking the age old question of "what bullet is best for my twist?" Instead of giving them guesses and hearsay. Why not give them the tools and knowledge they need for a logical answer?

[Eb1]

Sport45,

That was kind of a sarcastic point about the longer being heavier. As you can see in my last post. Longer is not always heavier. Compare the Hornady 75 grain A-MAX, 77 grain SMK, and the 75 grain Hornady HPBT.
The last two are shorter then the A-MAX. One being 2 grains heavier, and the other being equal..

dmazur quoted from wikipedia:what:: :)
For a given caliber, the heavier the bullet the longer the bullet will be

In the above post I mentioned three bullets. In which makes the above statement false.
The bullet is not always heavier if it is longer. Which is why the weight doesn't matter. Weight might matter because of the formula for weight? I would say that it is a small coincidence that weight is equated by mass(gravity). :)

[dmazur]

Eb1 -

My quote from Wikipedia was intended to provide an example of a long-useful empirical equation that had no mathematical proof until quite recently. I wasn't sure where you were coming from, and I was trying to emphasize that, as long as it works, maybe we shouldn't look at the tool too closely.

Your links mention the Greenhill formula but don't show how it was developed, other than a reference to assumed specific gravity. My point was, if that is assumed, and the formula uses both length and diameter (which is volume), then it is indirectly using weight. My second quote was from your link, not Wikipedia.

I have also read a lot of statements like "That twist rate won't stabilize a 105 gr bullet", and of course what may be overlooked is that the discussion is about bullets in .243 diameter.

And, yes, bullet shape can create a longer bullet in the same caliber that is lighter. That doesn't mean the Greenhill formula is invalid for estimating twist rates for a certain bullet length.

Using my previous example, it is like measuring flow in a ditch and saying Manning's equation is wrong because you measured a different flow rate than predicted. No, what is happening is that your assumed roughness coefficient doesn't match actual roughness...

My limited understanding of this is, if the formulas for twist show you are not even close, it is very unlikely a slight difference in bullet length is going to work magic. On the other hand, the formula might prove a useful guideline for telling the barrel manufacturer that you want 1:9 instead of 1:12 because you are planning to shoot "long for caliber" match bullets instead of varmint bullets.

The Greenhill formula -

Twist rate
For best performance, the barrel should have a twist rate sufficient to stabilize any bullet that it would reasonably be expected to fire, but not significantly more. Large diameter bullets provide more stability, as the larger radius provides more gyroscopic inertia, while long bullets are harder to stabilize, as they tend to be very backheavy and the aerodynamic pressures have a longer "lever" to act on. The slowest twist rates are found in muzzleloading firearms meant to fire a round ball; these will have twist rates as low as 1 in 60 inches (1,500 mm), or slightly longer, although for a typical multi-purpose muzzleloader rifle, a twist rate of 1 in 48 inches (1,200 mm) is very common. The M16A2 rifle, which is designed to fire the SS109 bullet, has a 1 in 7-inch (180 mm) twist. Civilian AR-15 rifles are commonly found with 1 in 12 inches (300 mm) for older rifles and 1 in 9 inches (230 mm) for most newer rifles, although some are made with 1 in 7 inches (180 mm) twist rates, the same as used for the M16. Rifles, which generally fire longer, smaller diameter bullets, will in general have higher twist rates than handguns, which fire shorter, larger diameter bullets.

George Greenhill, a mathematician at Emmanuel College, Cambridge, UK, developed a rule of thumb for use in calculating twist rates for a given lead-core bullet. The formula, named the Greenhill Formula in his honour, is:

Twist = (CD^2)/L * (SG/10.9)^0.5

where:

C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
D = bullet's diameter in inches
L = bullet's length in inches
SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

The original value of C was 150, which yields a twist rate in inches per turn, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 840 m/s (2800 ft/s); above those velocities, a C of 180 should be used. For instance, with a velocity of 600 m/s (2000 ft/s), a diameter of 0.5 inches (13 mm) and a length of 1.5 inches (38 mm), the Greenhill formula would give a value of 30, which means 1 turn in 30 inches (760 mm).

If an insufficient twist rate is used, the bullet will begin to yaw and then tumble; this is usually seen as "keyholing", where bullets leave elongated holes in the target as they strike at an angle. Once the bullet starts to yaw, any hope of accuracy is lost, as the bullet will begin to veer off in random directions as it precesses.

Conversely, too-high a rate of twist can also cause problems. The excessive twist can cause accelerated barrel wear, and also induce a very high spin rate which can cause high-velocity projectiles to disintegrate in flight. A higher twist than needed can also cause more subtle problems with accuracy: Any inconsistency within the bullet, such as a void that causes an unequal distribution of mass, may be magnified by the spin. Undersized bullets also have problems, as they may not enter the rifling exactly concentric and coaxial to the bore, and excess twist will exacerbate the accuracy problems this causes.


I rewrote the formula using Basic notation as the graphic didn't copy when I pasted it. The SG/10.9 term is equal to 1 if you use lead bullets, so the simplified formula is the left part only.

I believe the formula is only useful for estimating the twist necessary to stabilize a certain bullet. For the sake of discussion, let's say it is a "heavy" one. The formula does not give any hint as to the problems excessive spin can cause, such as bullets that aren't perfectly concentric. So, you can have a barrel that performs nicely with 70 gr bullets and doesn't like 55 gr bullets. It may not be that the bullet isn't stabilized, but that there is actually too much spin!

[dmazur]

Yes, you are absolutely right.

There are way too many statements about barrels with twist rate x not stabilizing a bullet which weighs y grains.

They are misleading. The length is what creates the lever arm, and the stability problem.

As long as we're trying to determine twist rate to stabilize lead bullets of different length, the Greenhill formula is useful, in its simplified form. And, if we're trying to see what twist rate might stabilize copper bullets (for example), we might need to drag in the second term and use the full equation.

But flat statements about bullet weight working/not working are not correct.

[depoloni]

Started out as a rant, reloado-political statement, or whatever...

Really glad you brought that up. I found it pretty informative and enlightening.

I'm not a competitive target shooter but I shoot a lot, been reloading for years but not as long as many (I'm 29). Admit that the weight vs. length issue as it pertains to twist rate is something I've never really understood - I've had some "weird" sucesses in experimenting that didn't fit the bill but now see a reason why other than "just lucky".

My old man's AR, when we tried it, has no problems spinning Sierra 65 SP's out of a 1-in-12 twist. Books say it shouldn't be so, and several others in that weight range tumbled. Probably a length thing. Same thing when I tried 160/162 bullets in my 7mm-08 T/C. Was told they wouldn't stabilize, some did some didn't as well but some did for sure. Again, probably an issue of being on the "twist fringe" where some similar weight bullets were shorter than others of the same weight.

Very cool. Thanks.

[Eb1]

dmazur, I wasn't being anything really about wiki-pedia. It is a long running gothca that comes from my work place, and there is no way you would have known that. We had a big todo (laugh) once when some quoted a wiki-pedia a fact during a project, and to find out it wasn't. So I am weary of using it for anything other than a private reference of sources to check myself. Not as a reliable resource.
No harm intended.

[NCsmitty]

Case capacity and the ability to be able to overcome the stability problem with velocity, does play a part in the equation.
For example, a 223 with a 1in9 twist barrel will have difficulty stabilizing a 77gr Sierra at 2800fps.
A 22-250, with the same twist barrel, will stabilize the same 77gr Sierra easily with 3300fps and the resulting higher rotational speed.

Lead-free bullets create their own issues with the longer but lighter bullets.

We've been over this many times before.


NCsmitty

[fguffey]

6mm Remington/244 Remington

F. Guffey

[Eb1]

fguffey, are you saying potato/potato?

[fguffey]

Forgive, now I see the question mark. Big expensive mistake, the 244 Remington came out with a 1-in-12 twist in 1955, about the same time the 243 Winchester came out with a 1-in-10 twist, the 243 did better with heavier and lighter bullets as in dual purpose, Remington in 1963 started over with a new rifle, same cartridge, called 6mm Remington with a 1-in-9 twist, but by that time the 234 was here to stay.

The 243 Winchester can not be chambered to 6mm Remington, as with the shorter 308 Winchester being larger in diameter than the 30/06 at the shoulder, the 243 is larger in diameter at the shoulder than the 6mm Remington, the 30/06 reamer will not clean up the 308 W chamber, the 6MM Remington reamer will not clean up the 243 chamber, the 243s parent case is the 308, the 6mm Remington's parent case is the 7X57.

F. Guffey