Twist Rates

[Lead melter]

OK, so I'm going to sound really dumb here, but can someone please explain the barrel twist rate/boolit diameter/boolit length/boolit weight/velocity ratio?

By this I mean how would one take into consideration the potential use of boolit "A" of weight "B" and diameter "C" and calculate a minimum and maximum usable velocity "D" and "E" taking into consideration the one constant, which is barrel twist rate "F"? Or does rotational spin "G" come into play also?

It thus far has escaped my mind how it is calculated.

I have seen the Greenhill formula, but it does not seem to apply in all cases. Or are the exceptions to the rule the proverbial "proof in the pudding"?

[Lead melter]

OK, so I'm going to sound really dumb here, but can someone please explain the barrel twist rate/boolit diameter/boolit length/boolit weight/velocity ratio?

By this I mean how would one take into consideration the potential use of boolit "A" of weight "B" and diameter "C" and calculate a minimum and maximum usable velocity "D" and "E" taking into consideration the one constant, which is barrel twist rate "F"? Or does rotational spin "G" come into play also?

It thus far has escaped my mind how it is calculated.

I have seen the Greenhill formula, but it does not seem to apply in all cases. Or are the exceptions to the rule the proverbial "proof in the pudding"?

[44man]

There is no foolproof method to predict what is needed. Almost all is based on experience of shooters.
The only guess is the slow twists work better with light, short boolits and faster twists work for the heavy, longer ones. There is a lot of bleed over and some boolits work for each twist.
Some guns like the Marlin .44 with the 1 in 38" twist will not shoot anything good. Most revolvers with faster twists will shoot almost any boolit but those with slow twists only shoot light boolits good.
The Greenhill never worked right even with the blackpowder loads it was figured for.
The velocity a boolit is shot with changes it all again and you can reach the point where a long boolit in a slow twist can't be shot fast enough to stabilize.
Your best bet is to post what you are shooting and it's rate of twist. You will get better answers.

[longbow]

I don't think your question has a simple answer but here is a site that will probably answer it.

http://www.nennstiel-ruprecht.de/bullfly/index.htm

This stuff gets a little beyond me (well, maybe more than a little). There can be many variables that come into play such as bullet density and design (spire point, tangent ogive, round nose, large meplat, smalle meplat, etc.) that make it difficult to apply the theory to the real world.

My understanding is that Greenhill came up with a more practical formula for real world applications.

Here is a quote from a website on Alfred George Greenhill:

"Greenhill applied this theory to give an account of the steadiness of flight conferred upon an elongated projectile by rifling. He determined the least angular velocity about its axis for which steady motion of a solid of revolution can be stable. ... This practical application of what was regarded as a recondite mathematical theory earned for him much renown at Woolwich."

Here is a Wikipedia page with good explanation of the Greenhill formula and how it applies(scroll down to "Twist rate"):

http://en.wikipedia.org/wiki/Rifling

Here are some other sites on the topic of twist:

http://www.loadammo.com/Topics/July01.htm
http://stevespages.com/page8e.htm
http://www.povn.com/4n6/Links.htm

For the most part, I doubt most of us would recognize that our barrels were out 1/2" in a 1:8.5 twist like David Tubb and the manufacturers use a compromise twist to suit a variety of bullet weights (lengths). Maybe a good example of too narrow a focus is .244 Remington vs. .243 Winchester where Remington used a slower twist that limited the useful range of bullet weights.

Anyway, I'm not sure if that helps or not but there is lots of info there to absorb.

Longbow

PS: While I was busy writing away 44man posted. I think these two lines sum it up nicely: "Almost all is based on experience of shooters." & "Your best bet is to post what you are shooting and it's rate of twist. You will get better answers." And so much shorter than my post!

[longbow]

I don't think your question has a simple answer but here is a site that will probably answer it.

http://www.nennstiel-ruprecht.de/bullfly/index.htm

This stuff gets a little beyond me (well, maybe more than a little). There can be many variables that come into play such as bullet density and design (spire point, tangent ogive, round nose, large meplat, smalle meplat, etc.) that make it difficult to apply the theory to the real world.

My understanding is that Greenhill came up with a more practical formula for real world applications.

Here is a quote from a website on Alfred George Greenhill:

"Greenhill applied this theory to give an account of the steadiness of flight conferred upon an elongated projectile by rifling. He determined the least angular velocity about its axis for which steady motion of a solid of revolution can be stable. ... This practical application of what was regarded as a recondite mathematical theory earned for him much renown at Woolwich."

Here is a Wikipedia page with good explanation of the Greenhill formula and how it applies(scroll down to "Twist rate"):

http://en.wikipedia.org/wiki/Rifling

Here are some other sites on the topic of twist:

http://www.loadammo.com/Topics/July01.htm
http://stevespages.com/page8e.htm
http://www.povn.com/4n6/Links.htm

For the most part, I doubt most of us would recognize that our barrels were out 1/2" in a 1:8.5 twist like David Tubb and the manufacturers use a compromise twist to suit a variety of bullet weights (lengths). Maybe a good example of too narrow a focus is .244 Remington vs. .243 Winchester where Remington used a slower twist that limited the useful range of bullet weights.

Anyway, I'm not sure if that helps or not but there is lots of info there to absorb.

Longbow

PS: While I was busy writing away 44man posted. I think these two lines sum it up nicely: "Almost all is based on experience of shooters." & "Your best bet is to post what you are shooting and it's rate of twist. You will get better answers." And so much shorter than my post!

[Tom Myers]

Try this one

http://www.uslink.net/~tom1/twistrate.htm

Tom Myers
Precision Ballistics and Records

[Tom Myers]

Try this one

http://www.uslink.net/~tom1/twistrate.htm

Tom Myers
Precision Ballistics and Records