bullet versus barrel twist

[Beekeeper]

I saw a formula for deciding the right boolit for a black powder rifle but lost the place so am interested in finding out if there is a formula that will give the (recommended best) boolit for a given barrel length and twist.

It is nothing important as I am just bored and looking for something to help as a mind game.Thought it might help me with some of the odd ball rifles I have.


beekeeper

[BCB]

Google The Greenhill Formula...

Should get you on your way...

Good-luck...BCB

[44man]

Google The Greenhill Formula...

Should get you on your way...

Good-luck...BCB
Doesn't work!
Then what BP rifles? Muzzle loaders, RB, inlines, BPCR?

[BCB]

What doesn't work?...

Google or the Greenhill Formula?...

Oh well, it was worth a try--either one I suppose!!!...

[mpmarty]

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CC0QFjAB&url=http%3A%2F%2Fkwk.us%2Ftwist.html&ei=FUJeUN-bC4P2igLX6oCIBA&usg=AFQjCNGzsb5YXPXoZApFsXKDD8e3Lgra7Q

[44man]

What doesn't work?...

Google or the Greenhill Formula?...

Oh well, it was worth a try--either one I suppose!!!...
Greenhill! It has been made to work by changing figures to match what is being shot so if you have a 1 in 10" twist, you can alter Greenhill to show it is correct.
Over the last 57 years I have collected about 100 different twist papers. All have been way too slow a twist for the cartridge.
Marlin used Greenhill to make the .44 a 1 in 38" twist.
There is nothing on paper that works better then experience of shooters.

[beagle]

The Greenhill formula was used originally for artillery. As stated, it's unsuitable for cast. There was a modified Greenhill formula published about 8 years ago in the Fouling Shot. It seemed to work pretty well but I've lost it. Maybe someone will post it here. Felix and I played with it over one winter with the .44 Magnum and various heavyweight (300+ grain) bullets (all designs that we both had and could beg, borrow or steal. I made a spreadsheet model and we punced in the weights, lengths and diameters for all variables we could imagine. The conclusion we reached is that the .44 Magnum required a twist of 1 to 26 3/4" for the probability of good accuracy with 240-320 grain cast bullets. Felix was shooting a Ruger and his 1:26 twist was fine. I played with a factory M1894 .44 Mag, a 336 .44 Mag (also 1:38) and a M1894 with a 1:20 twist custom barrel. The 1:20 twist handled 300+ bullets fine but was unusable on lighter bullets. I bought a new Winchester Legacy with a 1:26 twist and had immediate success with it so the modified Greenhill was pretty close to right.

It also worked for me with the M1894 Marlin .25-20 with its 1:14 twist. Rebarreling to a 1:12 twist brought 95 grain bullets into stability and good accuracy.

Maybe somebody will post it./beagle

[noylj]

I have always wondered if anyone EVER really did a study of handgun twist rates or if the twist rate grabbed out of thin air way back when has just always been used since nobody expected any sort of real accuracy from a handgun.

[44man]

I have always wondered if anyone EVER really did a study of handgun twist rates or if the twist rate grabbed out of thin air way back when has just always been used since nobody expected any sort of real accuracy from a handgun.
I have fallen in love with BFR revolvers because all have faster twist rates. Never have I shot such tight groups and out shoot most rifles to 547 yards.
I even believe a 45-70 rifle should be 1 in 16". 1 in 20" is too slow and 1 in 18" is borderline. My BFR 45-70 is 1 in 14".
I think the .44 mag would be better at 1 in 16".
A rifle that can shoot faster can use a slower twist with revolver calibers.
Beagle showed that but the 1 in 38" sucked big time. Even Marlin changed the .444 to 1 in 20" because of complaints but left the .44 mag the same, based on Greenhill. You can change any formula for one caliber but it will fall apart with another.
Magnum Research is getting it right for revolvers.
My thoughts on revolvers is that as the barrel gets shorter and velocity gets lower, the twist should be faster.

[HORNET]

The SIMPLIFIED Greenhill Formula is:
Twist = 150 X D²/L
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
The biggest problem with this version is the 150 constant. The reason that it's called the "simplified" formula is because ALL the other factors in the FULL Greenhill formula were solved for the specific instance of a 220 grain roundnosed jacketed bullet at .30-40 Krag velocities and simplified into a 150 constant. It works moderately well UNDER THE CONDITIONS that it was derived for and less well as you get further from those conditions.
Round ball at 700 fps? Not a real accurate answer.
Cast spitzer at 1400 fps? Probably not gonna be exact.
Solid copper VLD at 3700 fps? Also not real accurate.
311290 out of a .30-30? Probably real close to giving you the required twist.
The FULL Greenhill is very accurate and includes factors for the changes in the ballistic coefficient as the velocity changes, the initial velocity, and enough other variables to give you a headache. That's the reason the simplified version was developed. It's a Rule-Of-Thumb that people insist on trying to apply incorrectly.

[swheeler]

BK try this http://kwk.us/twist.html

[44man]

BK try this http://kwk.us/twist.html
Gives me 1 in 32" for a RD 265 gr .44 boolit---WRONG.

[BCB]

I remember reading someplace—might have been the Fouling Shot—that the constant can be changed to sort of fit the boolit and velocity. I don’t remember where it was at, but that might be a help to using the Greenhill Formula…

BCB

[swheeler]

Gives me 1 in 32" for a RD 265 gr .44 boolit---WRONG.

I take it you mean for the velocity you can reach in your revolver a 1-32 twist won't stabilize the 265RD.

[44man]

Lets go to the .500 JRH. It shows 1 in 37"----WRONG. Shoots best at 1 in 15".
Nothing I ever found works.
To under stabilize is far worse then over. Over will go to sleep with distance. Just right will work at many distances.
Notice a gyroscope will start to wobble as it slows and if you touch it it goes crazy and falls.
Look at a .50 caliber muzzle loader. 1 in 60" will stabilize a RB but not a Maxi ball. 1 in 28 works for neither but works for a sabot and a sub caliber bullet. ! in 48" will do well with both a RB and a Maxi ball.

[44man]

I take it you mean for the velocity you can reach in your revolver a 1-32 twist won't stabilize the 265RD.
Yes but not even in a rifle. A rifle should be somewhere from 1 in 20" to 1 in 26" depending on the boolit.

[swheeler]

44; I just plugged in the numbers for the 360 minnie ball that I shoot in a CVA 1in 48 bbl. It shows a 1 in 41 twist is needed to be fully stabilized at exit with as cast length of .840" and pure lead for SG at 1430 fps. Now this minnie shoots good in my rifle at 50 yds, much past that I can't verify because of my eyesight and the sights, but I did clobber a WT doe with it at about 100 yds, maybe a hair over. Now this minnie ball will shorten some when it obturates the bore, how much I don't actually know, just for grins I figured .050" that calculation shows a 1 in 44 twist is the min needed to full stabilize, so according to that my 1in48 is not fully stabilizing the minnie.
I also used this calculator for CM in carcano 91/41, Jeff NZ did too. It recommended a velocity of 1750 fps to fully stabilize that bullet, sure showed an improvement, no more perfect ovals on target that were there at 14-1500. I'm sure there is no perfect way to estimate twist rate, but this one seems better than Greenhills basic formula.

[44man]

You can find a match if you look hard but the problem is that the formulas do not fit every caliber or bullet.
That calculator is close for some calibers. It is far out for others.
I would not depend on it. But it really is better then Greenhill.
Distance shot is also so important.

[HORNET]

Repeat: The further you get from the conditions that were simplified in generating the 150 constant in the simplified Greenhill, the less accurate the results are. If you want something more accurate, it's going to be more complex. I believe Tom Meyers has a twist calculator on his website or you could try this one: http://www.border-barrels.com/barrel_twist.htm
Lots of fun to play with.

[Iron Mike Golf]

44Man, what do you think of this sort of calculator?

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[BCB]

My Ruger SRH 44 Magnum has a 1in 20 twist according to literature. It stabilizes a 429650 (~320 grains) well as can be verified by some groups I have shoot at 100 yards from a good sandbag rest…

But the Ruger Carbine has a 1 in 38 twist according to literature. I wonder why Ruger went to such a slow twist with the rifle?...

BCB

[noylj]

BCB:
higher velocity.
Any calculation has to take into account, I would think, the caliber, the bearing surface, the bullet weight, the muzzle velocity, and who know what else.

[303Guy]

Not muzzle velocity apparently. A slower twist may simply be better for higher velocity from a rifling shear (or skid) point. A faster twist is required for transonic velocities which is likely in a hand gun. The Ruger carbine is gas operated so transonic muzzle velocities wouldn't be indicated for that rifle.

[44man]

44Man, what do you think of this sort of calculator?

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
I don't know. I fed in my .500 and I get a stability answer I can't interpret.

[44man]

My Ruger SRH 44 Magnum has a 1in 20 twist according to literature. It stabilizes a 429650 (~320 grains) well as can be verified by some groups I have shoot at 100 yards from a good sandbag rest…

But the Ruger Carbine has a 1 in 38 twist according to literature. I wonder why Ruger went to such a slow twist with the rifle?...

BCB
It was Greenhill. I read somewhere where tests with a .44 rifle did best at about 1 in 22" to 1 in 25" with added velocity.
With a faster twist rate it is easy to just reduce velocity but when the twist is too slow you can not get the boolit fast enough.

[Iron Mike Golf]

I don't know. I fed in my .500 and I get a stability answer I can't interpret.

What was the stability result you got?

[Iron Mike Golf]

44man,

Look at this article: http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

I skipped the math. In Section III, there's a couple of case studies. This tool looks useful. I know their focus is on BR rifles.

[beagle]

That was my thinking also and I went to the trouble and expense of having a M1894 rebarrelled with a Douglas 1:20 twist as I had had no stability problems with the Rugers. In the Marlin, it handled 300 grainers fine but drop to a lower rate and they were all over the place so that's not the answer. I know, I paid dearly for that piece of education.

Face it, the .44 Mag rifles are a pariah. The best you can hope for is acceptable accuracy with a variety of bullets by smart sizing and bullet hardness. The variety of weights and length we have on bullets is just too much to accomodate with one twist.

Now, if some genius comes up with a dial-in twist to set for each bullet weight, we'd have a going concern./beagle


My Ruger SRH 44 Magnum has a 1in 20 twist according to literature. It stabilizes a 429650 (~320 grains) well as can be verified by some groups I have shoot at 100 yards from a good sandbag rest…

But the Ruger Carbine has a 1 in 38 twist according to literature. I wonder why Ruger went to such a slow twist with the rifle?...

BCB

[44man]

What was the stability result you got?
It was 11.837. What does it mean.

[KCSO]

Greenhill is just a starting point. Then you have to take into account bullet shape and the velocity you are going be pushing the bullet. Depending on velocity you can stabalize a cast 110 gran bullet in a 30 caliber barrel at several different twist rates.

[44man]

44man,

Look at this article: http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

I skipped the math. In Section III, there's a couple of case studies. This tool looks useful. I know their focus is on BR rifles.
The math is like looking for dark matter in the universe! :kidding:
I suck at any math let alone that stuff.
I look at it this way, mechanical only. Shoot close and use a slower twist to eliminate rotation around the flight path from over spin. Shoot far and ignore over spin so the boolit goes to sleep with range and is accurate way out there.
Forget the transition point stuff from supersonic to subsonic, if spin is enough the boolit will still shoot far.

[montana_charlie]

My BFR 45-70 is 1 in 14".
What is the length of the bullet you shoot in that revolver?

CM

[44man]

What is the length of the bullet you shoot in that revolver?

CM
Doesn't seem to matter. These are all accurate.

[Iron Mike Golf]

The math is like looking for dark matter in the universe! :kidding:
I suck at any math let alone that stuff.
I look at it this way, mechanical only. Shoot close and use a slower twist to eliminate rotation around the flight path from over spin. Shoot far and ignore over spin so the boolit goes to sleep with range and is accurate way out there.
Forget the transition point stuff from supersonic to subsonic, if spin is enough the boolit will still shoot far.

Yeah, I hear you. I agree with your conclusions/rules of thumb.

That 11+ stability would indicate "overstability". I think ballistician's argue about whether that is good, bad, or doesn't matter. I'd call anything over 1.5 good to go. I'd want to run the numbers for worst case air density (low temp, etc)