Twists, Twists, and more Twists

[roundball]

Does anyone happen to know of a definitive, authoritative source regarding optimum twist rate for a given diameter round ball by caliber?

The reason I ask is that I see some posts making across the board statements that such & such is a fast twist...and so & so is a slow twist...without any other qualifying information, yet I don't believe such across the board statements are correct for all round balls in all calibers.

For example, one might claim that a 1:48" is a fast twist for a PRB without referencing any caliber, yet I know that at least one compay made a match grade .40cal RB rifle with a 1:48" twist, suggesting to me that 1:48" was indeed the optimum RB twist for a .40cal PRB...(and I personally know it's extremely accurate for the .45cal).

On the other hand, I can speculate that the large size .58cal ball may do better with a slower twist such as 1"66" or 1:70".

My question is, putting aside speculation and anecdotal supposition, does anyone know of an existing bone fide standard for RB twist per caliber by an established industry source such Lyman, etc?

 

[jtmattison]

There is a formula to use to figure what length of projectile a rate of twist in a particular caliber will stbilize but I can't find it on my computer. I remember posting it on here before.

HD

 

[jtmattison]

I found it:
Twist thread

Look for my post.

HD

 

[roundball]

I found it:
Twist thread

Look for my post.
HD

Yes, I know about the Greenhill formula but isn't it designed for longer than caliber projectile bullets, right.....even requiring a specific ballistic coefficient, etc

 

[roundball]

I found it:
Twist thread

Look for my post.
HD

Yes, I know about the Greenhill formula but isn't it designed for longer than caliber projectile bullets, right.....even requiring a specific ballistic coefficient, etc

Here is the Greenhill Formula...it's based upon a "specific gravity", not ballistic coefficient...
[url] http://www.loadammo.com/Topics/July01.htm[/url]

 

[BS]

Twist = 150 X D2/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

Basiclly, it is 150 x the dia.

150x.320= 48
150x.500= 75

 

[roundball]

Twist = 150 X D2/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

Basiclly, it is 150 x the dia.

150x.320= 48
150x.500= 75

Right...I understand the Greenhill formula.

 

I think that anything less than 1 in 48 would be a fast twist. In rifles twists faster than that generaly are for conicals, and shorter barreled pistols. I don't think there is a specific twist rate that is specific to deliniate the two. And we know that there are people on the Forum that think 1 in 48 is too fast a twist :rotf: Names should be changed to protect the guilty. :blah: Bill

 

[BLAHMAN]

The guy to ask would be Ed Rayl....I think Olie has the formula that Ed uses. He gave me that formula once....danged if I can find it.

Hold on there....I think it's the same place I put my hair.....

Sucks getting old. There are eight things that happen to a fella that tells him he's getting old.

The first is memory loss

....give me a week or so and I might be able to remember what the others are! :haha:

 

[roundball]

Some Examples of round ball twists:

.32/.36/.40cal
GM = 1:48"
TC = 1:48"

.45cal
GM = 1:60"
TC = 1:66"

.50/.54cal
GM = 1:70"
TC = 1:66"

.58cal
GM = 1:70"

Has to be some sort of relationship between caliber and twist rate.

 

[Slamfire]

My Numrich .350 bore has a 1 in 40" twist. :winking:

 

[Zonie]

Others have mentioned that the Greenhill Formula is designed for elongated bullets and I agree.

It is based on the idea that a elongated bullet needs to be spinning at or above a certain number of revolutions per minute to give it the gyroscopic stability to prevent yaw or tumbling (keyholing).

As a tumbling roundball or one which is undergoing yaw would not change its crossectional shape relative to the wind, I don't think the Greenhill Formula applys to them.

Now, if we could find someone who is into demonology we could find out just how fast a roundball has to spin to throw off the demons of inaccuracy! :grin:

 

[BS]

Others have mentioned that the Greenhill Formula is designed for elongated bullets and I agree.

It is based on the idea that a elongated bullet needs to be spinning at or above a certain number of revolutions per minute to give it the gyroscopic stability to prevent yaw or tumbling (keyholing).

As a tumbling roundball or one which is undergoing yaw would not change its crossectional shape relative to the wind, I don't think the Greenhill Formula applys to them.

Now, if we could find someone who is into demonology we could find out just how fast a roundball has to spin to throw off the demons of inaccuracy! :grin:

The Greenhill Formula looks just fine for hunting loads. Maybe a little slow for target shooters, you might need to speed it up some. :grin:

 

[Slowpoke]

There isn't isn't one.... if you're looking for "absolute truth" on the matter, I believe you will never find it. Most people are not educated enough (me included for sure) to understand ballistics and I believe this is what leads to across the board statement such as "blank" is a slow/fast twist.
Personally, I believe it is sole based on caliber/ball size and the ability to stabilize the ball. My 25cal has a 32 twist and my 54 has a 80 something. Why? the smaller the object the faster the spin is needed to keep it stable.... earth vs a top... blah blah blah. Probably not what you're looking for but just my 2 quid.

 

[roundball]

There isn't isn't one.... if you're looking for "absolute truth" on the matter, I believe you will never find it. Most people are not educated enough (me included for sure) to understand ballistics and I believe this is what leads to across the board statement such as "blank" is a slow/fast twist.
Personally, I believe it is sole based on caliber/ball size and the ability to stabilize the ball. My 25cal has a 32 twist and my 54 has a 80 something. Why? the smaller the object the faster the spin is needed to keep it stable.... earth vs a top... blah blah blah. Probably not what you're looking for but just my 2 quid.

I understand...maybe the place I need to poll are the manufacturers themselves...SOMETHING causes companies like GM, TC, to decide what twist to use when they make RB barrels for a particular caliber.

As an interesting question...wonder what causes TC to use 1:66" and GM to use 1:70" twist for the same caliber?

Can't imagine there's any noticeable difference in the end results from a 1:66" vs. 1:70", yet in some cases we see that they each arrived at a slightly different twist for the same caliber RB, had the rifling machines built, began production, etc.
:hmm:

 

[jtmattison]

No telling where they come up with it. I remember reading somewhere that 1:60 was a "fast twist" and something about roundball guns with 1:80 to 1:120 twist.
I guess if it works, go with it.

HD

 

[arcticap]

I tend to think that barrel length might also have an effect on the "best" rate of twist needed to stabilize a PRB. I recently read a report about a scoped .50 caliber pistol that shot a 2 inch PRB group @ 50 yards with a 1 in 28 inch twist barrel. Nothing may be optimal about that rate of twist without some consideration of the barrel length variable that's involved.

 

[roundball]

I just received a response back from my contact at GM:

"...that they are not aware of any known scientifc industry approach used to decide on twist rates for MLs..."

"...that GM made their twist rate decisions based upon existing twist rates per caliber that seemed to have already settled into place within the ML industry..."

 

[jtmattison]

I just received a response back from my contact at GM:

"...that they are not aware of any known scientifc industry approach used to decide on twist rates for MLs..."

"...that GM made their twist rate decisions based upon existing twist rates per caliber that seemed to have already settled into place within the ML industry..."

There you go. There is no science to it in the industry.
So we could invent our own twist rates and label them what we want.

"Fast twist" = from 1:5 to 1:50
"Medium twist" = from 1:51 to 1:66
"Slow twist" = from 1:67 to 1:200

There you go. The Huntin Dawg standard for rifling twist rate in muzzleloaders.

HD

 

[roundball]

:grin:
I think you should start a new thread declaring that the "Huntin Dog Twist Rate Chart" is now in place for the ML industry's reference.