Optimum Twists By Diameter

[Good Cheer]

Has anyone ever done a study or published data on optimum twists for round ball by diameter?

There's a lot of anecdotal info, hearsay, conjecture and such. Has a compilation of data ever been published?

[KCSO]

Greenhills Formula is what you are looking for.

[quail4jake]

Greenhill formula, round balls land at slow pitch due to short length 1:66" is common. I'm interested to hear from anyone how much bore diameter influences the ideal pitch for a round ball.

[StolzerandSons]

These are the two formulas that are most commonly used for roundball twist rates.
Ed Rayl formula
Pi (3.1415) times bore diameter divided by .02618.
example: (3.1415 x .500) / 0.2618 = 59.99 or 1:60 twist for .50 caliber

Cox Formula
Pi (3.1415) times bore diameter divided by .029.
example: (3.1415 x .500) / 0.29 = 54.16 or 1:54 twist for .50 cailber

As for studies or published data...no single study that I know of but a couple hundred years of historical proof in use seems to make the formulas a proven. Rayl's is obviously newer since he is still alive, Cox's formula is older but I don't think even he invented it since the common twist rates for most calibers were around long before he published his formula.

[quail4jake]

Wow! That's great to know, that will be recorded in my useful ballistics book!

[curator]

Try putting your data into the greenhill calculator. Both formulas mentioned give a "ball park" figure on minimum twist but do not factor in velocity which does matter. Slower velocities demand faster twist to stabilize a round ball. This is why muzzle loading pistols barrels are rifled with a faster twist rate. Here is a link to a calculator site: http://kwk.us/twist.html

[rodwha]

When entering numbers for the equivalent of a .45 Colt the velocities drop to unreasonable numbers to make a 1:16" twist work. Is there a different number/calculator needed for handguns? And for a lead ball to be stable in my ROA a velocity of 101 fps is needed.

[curator]

rodwha:

The "calculator" only shows the MINIMUM twist for projectile stability at the entered velocity. Keep in mind that rotational "spin" does not slow down like forward velocity. To be stable enough to provide accuracy a bullet must retain sufficient spin as velocity is reduced. Otherwise accuracy at any distance degrades quickly. This necessitates starting the bullet with sufficient twist to retain stability at longer distances with much slower velocity. Bullets that are too long for the rifling twist are often reasonably accurate at close range but accuracy deteriorates quickly as the range increases. Contrary to popular opinion, a homogeneous, balanced bullet spinning around its center of mass is almost impossible to "over-stabilize." Of course, stripping out of the rifling can occur, negating all that!

[StolzerandSons]

There are a couple reasons why the Greenhill formula doesn't work for roundball:
The Greenhill formula you find on the internet and in most books is usually the very shortened and simplified version of a much more complicate formula. This is what you usually find: T = 150 * D^2 / L .

1. The complete Greenhill formula assumes the air is frictionless, so it ignores any boundary effects such as turbulence. The formula also assumes that the density and shape of the projectile are consistent and flawless.

2. The mathematical model Greenhill used for the bullet is a "prolate spheroid" - a round shape where the length is longer than the diameter.

3. The artillery shells Greenhill was modeling in his work were about 2.5 times longer than their caliber, and all of the experimental data used to calculate the value of the constant in the simplified formula (usually given as 150 or 180) are from projectiles whose length was between 2.5 and 8.0 times longer than their caliber.

4. Since Greenhill chose to model the projectile as a prolate spheroid, many of his assumptions and simplifications don't work for round balls. The complete formula gets strange when the projectile length is less than about twice as long as the caliber. And when the length/caliber ratio gets down to 1:1 (as in a round ball), the full formula fails to function entirely, some terms shrink to zero, while other terms go to infinity, so the formula gives no result.

The link that curator posted is not an actual Greenhill formula calculator since the simplified Greenhill formula gives no variable for velocity. If you want to get close to making the simplified Greenhill formula work for roundballs you can substitute 111 in place of the 150(or 180) value that is commonly used.

Greenhill generally works pretty good for bullets but it falls apart when using it for roundball. The simplified Greenhill equation is at best a happenstance that gets close sometimes with roundball.

[Hanshi]

Formulas also fail to take in to consideration the depth of the rifling as it relates to twist. Along with that omission is the fact that each barrel is a unique case unto itself.

[kens]

Has anyone ever done a study or published data on optimum twists for round ball by diameter?

There's a lot of anecdotal info, hearsay, conjecture and such. Has a compilation of data ever been published?

That is pretty easy, there has been muzzle loaders in use since the 18th century, and by now there is plenty of empirical data.
.32 cal RB 1:48 is good
.45 RB 1:66 is good
.50 & .54 RB 1: 66 is good.
.58 RB 1:70

here is the link to the 3 major barrel makers, you can see their twist rates. no need for additional math, they are all good barrels.

http://www.colerainbarrel.com/rifled_barrels.html

http://ricebarrels.com/chart.html
http://www.gmriflebarrel.com/black-p...oader-barrels/

[rodwha]

Initially I created two cal and ball projectiles assuming my NMA would have the slow twist that was common (Pietta). One is a .400" 170 grn WFN and the other is a .460" 195 grn version. I found my NMA actually has something like a 1:16" twist as my ROA does. I also found that my more accurate (and useful for hunting) powder charges stayed the same despite what projectile I used and so now I'm contemplating filling the excess chamber space with lead. But I'm also considering that one day I'll have to have an Uberti Remington revolving carbine and would want my bullet (assuming it has a twist that will work well) to work well out to 75-100 yds for hunting. As I've seen chronographed results from said rifle with the same powder (Olde Eynsford) and similar bullet I see that it gets about 1075 fps with a 240 grn bullet with 30 grns of 3F. However I'd want to ensure that it would be stable out that far.

[vzerone]

Something to think about. Many of us have shot round balls from are modern centerfire rifles using squib loads. I don't know about the rest of you, but I was pleasantly surprised at how accurate they shoot. The twist rate in 30 caliber be anywhere from 1-10 and up. In the 7.65 you're looking at twist in the 9's. Me thinks that those real slow twists back in day were with concerns of making it easier to load a ball in those fouled bores along with not holding as much fouling as a faster twist would.

[waksupi]

That is pretty easy, there has been muzzle loaders in use since the 18th century, and by now there is plenty of empirical data.
.32 cal RB 1:48 is good
.45 RB 1:66 is good
.50 & .54 RB 1: 66 is good.
.58 RB 1:70

here is the link to the 3 major barrel makers, you can see their twist rates. no need for additional math, they are all good barrels.

http://www.colerainbarrel.com/rifled_barrels.html

http://ricebarrels.com/chart.html
http://www.gmriflebarrel.com/black-p...oader-barrels/

That has it covered quite well.

[mooman76]

There is nothing published because one optimum twist per caliber doesn't exist. If it did then everyone that made barrels certainly by now would know which twist is best per caliber. Many twists will work or can be made to work. Many other things go into play such as already mentioned, depth of rifling and velocity used or wanting to obtain. The slower the twist, the more velocity that is needed to keep the RB stable. So if you want to shoot long distances you need higher velocity to keep the ball stable as it slows over that distance. But if you are shooting shorter distances and want to use less powder a faster twist will work better for you.

[kens]

There is nothing published because one optimum twist per caliber doesn't exist. If it did then everyone that made barrels certainly by now would know which twist is best per caliber. Many twists will work or can be made to work. Many other things go into play such as already mentioned, depth of rifling and velocity used or wanting to obtain. The slower the twist, the more velocity that is needed to keep the RB stable. So if you want to shoot long distances you need higher velocity to keep the ball stable as it slows over that distance. But if you are shooting shorter distances and want to use less powder a faster twist will work better for you.

I'm not quite sure about that.
Please tell me where to buy such a barrel as to need more velocity to stabilize a RB.
I shoot a 1:66 .45 rifle, and I do not need velocity to stabilize it. Is there a .45 barrel available that is slower than 1:66? I thought I already had the slowest available? and it shoots just fine

[John Taylor]

Forsyth came up with the idea of a large bore with slow twist and lots of powder. Twist was 1/4 turn in the length of the barrel. Problem is it takes a lot of powder to get it to shoot, maybe 200 grains in a 58 cal. Rifling had wide grooves and narrow lands.

[oldracer]

The post by Kens is what my mentor told me for round balls. The twist rate can always be faster since the object going down the barrel is a ball. If you start elongating what you are shooting then things get much more complicated. My 50 caliber Douglas stainless XX barrel is 1 in 48 and both round balls and Mini's shoot exactly the same at 100 and 200 yards using 80 grains FFG!

[DIRT Farmer]

When you get into target guns you find a lot of varaibles. There was one shooter that I knew who shot a round ball bench gun 50 cal with 250 grains of ffg with a twist of one turn in 14 feet. He collected a lot of medals. Another who shoots small shiolette and chunk who wanted lower recoil in his 40 and insted of the standard 1-48 went with 1-20. He also won a lot of medals. My 1-48 40 cal barrel dosent care about the powder charge as long as its between 40 and 65 grains of fffg as long as I load a 400 ball and 20 thousand patching. Go figure

[oldracer]

John Taylor said 200 grains of powder.....I bought a very nice under hammer in 58 caliber that the owner had tried to shoot it with 200 grains of powder and it nearly dislocated his shoulder he said? If I tried that much powder it would cause my dentures to fly!!!!