So, in a nut shell if it has a slower twist, you will have to shoot it faster for any given weight of boolit, than a faster twist barrel to get it to stabilize.
For instance.
Bullet A out of 1-14 barrel 1900fps
Bullet A out of 1-10 barrel 1700fps
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So, in a nut shell if it has a slower twist, you will have to shoot it faster for any given weight of boolit, than a faster twist barrel to get it to stabilize.
For instance.
Bullet A out of 1-14 barrel 1900fps
Bullet A out of 1-10 barrel 1700fps
In a nutshell, yes.
History will record, with the greatest astonishment, that those who had the most to lose, did the least to prevent its happening.
The problem with Liberals is not that they're ignorant, it's just that they know so much that isn't so.
- Ronald Reagan
Excellent!! .... Thanks for posting!Originally Posted by c3d4b2
Eutectic
I have not had a chance to read this whole thread, but here's the little bit that I've read about balanced flight. It's not exactly coming from an unbiased source, but perhaps it's spot on?
Balanced Flight
It is clear that a projectile being able to retain stability throughout its flight will go farther and will be more accurate. Warren Jensen, partner and designer at Lost River Ballistic Technologies® identified the conditions that could be translated into projectile design, which would exhibit very long distance accuracy. The concept is called Balance Flight and is patented (6,629,669).
Working with PRODAS software, Jensen designed projectiles in multiple calibers, where the linear drag on a projectile is matched to its rotational drag. In other words, forward rate of deceleration and an axial rate of deceleration are balanced. The gyroscopic stability remains constant resulting in the projectile remaining on its original trajectory path. Non-balance flight occurs when a projectile’s spin is too great, thus leading to an "over spin" of the projectile. Over spin leads to increasing angles of repose, which result in increased drag and can result in instability at the transonic threshold.
An important component of Balanced Flight is the design of the barrel lands and grooves. A ratio of a total surface of the projectile to a total surface area of the physical feature must be in the range of 3.00:1 to 4.00:1. As a result of projectile and barrel land / groove design, the drag coefficient is reduced to a range of 0.100 to 0.250. In addition, the bearing surface of the projectile has a depth equal to 1% of the caliber of the projectile and a ratio of a total surface area of projectile to the total surface of the physical feature in the range of to 3.00:1 to 4.00:1. The purpose is to impart an ideal axial surface friction upon launching, which during flight produces a trajectory characterized by a continuously decreasing rate of axial deceleration.
Balanced Flight is best associated with the .408 CheyTac® caliber. However, Jensen first designed projectiles in 30 caliber, which resulted in major
improvements over existing projectiles in this caliber.
Balanced Flight’s improvements of projectiles in the 40 caliber went far beyond expectations. Could a Balanced Flight improve the 50 caliber projectiles? The answer is yes, but it would require specific changes to standard 50 BMG land, groove, and twist configurations. Combined with the CheyTac® cartridge case, the .408 projectile has found an ideal launching platform. This platform represents the best long-distance cartridge to date – in all ballistic-characteristic, including kinetic energy.
http://www.cheytac.com/WhitePapers/C...paper%20V2.pdf
Dannix
That is very interesting stuff. I recently built a 22 cal rifle with a 6.5" twist. It was built for 90gn vld bullets. One big difference with this Walther barrel is that it has no lands or grooves, it has a polygonal interior with the 6.5" twist. I wanted to try this type barrel to see if it would foul less than conventional rifling. Now after reading this I wounder what effect if any it may have on drag.
Still makes my head hurt!
Maybe we should shoot round balls! I grew up with round ball muzzle loaders and built many of them. I always read they go to pot after so far and it is the reason for the fast twist inline rifles, so you can shoot farther. Actual advantage to the bullet shooting guns is trajectory, it is flatter.
Take my .54 Hawken. I use 100 gr of powder for normal shooting to 100 yards and it is extremely accurate. It is still accurate at 200 meters but to get a flatter trajectory I go to 120 gr. Add sight adjustment and I hit 4 out of 5 chickens off hand at 200 meters and the gong is just too easy.
The twist is 1 in 60". I do not know just how far the ball will maintain accuracy but I suspect until it hits the ground.
Now take a 1 in 48" twist and a round ball. Reducing the charge a little and it still shoots great but a Maxi ball will also shoot great with the same charge so we are getting into boolit stability too. The 1 in 60" twist will spray boolits.
The super fast twists of inlines means bullets only but if you just pop out a round ball, I suppose it will be decent.
So the over spin on a round ball does not work and the bullet under spun does not work either.
To be practical, we work loads for each projectile to find the velocity where it is accurate. That relates to spin and stability but also relates to the range to be shot.
Too much spin is as bad as not enough. Barrel length does enter into it because of the velocity you can achieve so I feel the shorter the barrel, the faster the twist should be. Could this be why the BFR revolvers work so well? I would love to try a 1 in 16" twist with my 45-70 BPCR too because velocity is limited.
We see all the math and design of special bullets but just how does it relate to cast boolits?
The super fast twists of inlines means bullets only but if you just pop out a round ball, I suppose it will be decent.
So the over spin on a round ball does not work and the bullet under spun does not work either.
Not quite the case. To use 44mqan's example of the RB in a ML I shoot RBs in a TC Black Mountain Magnum .50 cal. It is a side lock not an inline but it is made for 150 gr Pyrodex charges and saboted bullets so it has the 28" twist. I worked up a very decent RB load at 1950 fps using FFFG Triple 7. They would shoot into 1 - 1.5" at 50 yards (shot several 5 shot groups. I then moved the target to 100 yards to get a zero and the groups were 16"+! I thought I'd done something wrong so I repeated the 50 yard test; same thing, 1 - 1.5" group. Moved the target back to 100 yards and the same thing; a 16+" group. I slowed the RBs down and did the testing at 100 yards. At arond 1650 fps I was getting 2.5 - 3" at 100 yards. Moved the target to 50 yards and shot a 1.5" group. MaxiBalls (cast of 1-16 alloy) shoot into 2 - 3 moa at 100 yards at 1570 fps.
The same thing can be observed with other bullets, especially cast bullets. If the bullet is severely imbalance the group dispersion will not be limear as range increases. As an example a test I conducted for Bass was comparing M118 bullets with unbalanced (purposely) M118 bullets. Ten shot groups were fired with at 50, 100 and 200 yards. The rifle has a 10" twist and velocity was 2666 fps. The balanced bullets shot into groups with linear expansion of group size. The unbalanced groups showed a definate decrease in accuracy and non - linear expansion of groups as the range increased.
This also applies to cast bullets when we drive them above the RPM threshold. The same test was also done with 311291 at 1912 fps and at 2487 fps. The slower 1912 fps load was below the RPM threshold and gave 10 shot groups of .7" at 50 yards, 1.55" at 100 yards and 2.5" at 200 yards. This obviously shows a linear expansion of the groups and good stability. The 2487 fps gave 10 shot groups of 2.55 at 50 yards, 4.7" at 100 yards and 14.5" at 200 yards. Obviously the linear expansion was greater with the faster and more unstable load that was over the RPM threshold.
This test is easily reproduceable and predictable. It is also the reason not to test at 50 yards with rifles if pushing over the RPM threshold. It is also a good reason to test at the maximum range to be shot when confirming what appear to be "accurate" shorter range tested loads.
Bottem line; a RB can be over stabilized as can be any other bullet; jacketed or cast.
Larry Gibson
Larry, you said the same thing I did. Slow the round ball in a fast twist and it does OK. However, I like better accuracy from the ball.
This is what I want at 50 yards for 5 shots and no more then double that at 100.
By the way, that is with open sights.
Last edited by 44man; 11-20-2010 at 01:56 PM.
Reading Larry's post again shows we are in total agreement for much of the discussion so I don't understand the "not quite the case" wording????
Larry
A bullet or boolit understabalised will do the same thing. I had an experiance with a 38/55 with 1/14" twist shooting 330gn boolits with a muzzle velocity of 1400 fps.
I wont bother going out to measure them for length but suffice to say that with the 1/14" twist at 1400 fps they were marginally stable.
I was preparing for a long range gong match and felt I had developed a really good load. It was grouping very well at 100, 200, and 300 yards. I dont have actual group sizes at each range but it was about proportional to the range increase.
Then surprise!!! past 300 yards they began to have an audible buzz in flight. At 500 yards I was doing good to keep them in the same hay field with my target, SHOCK!
The fix was a slightly shorter boolit. With the shorter boolit at about 300gn everything stayed together nicely and the gun/load did well at the shoot to the longest range we shot, 800 yards. The lesson for me was just because a load does good at moderste range do not assume it will continue to do good at longer range. Shoot to be sure.
In what I have read from the old masters of the 1890's and early 1900;s they seemed to prefer the absolute minimum spin for the range they were shooting, and steared clear away from more than enough spin.
Disclaimer: I am not an expert.
First question(SWAG answer) : Think about a top. You start spinning and it and it wanders around a center point for awhile, settles down and stays on its axis, then goes to unstable. I think this is called precession and you might google it and come up with some good information.
Second Question: The rpms at 530 yards will be mostly independant of the velocity. Once the bullet exists the muzzle, the only angular forces acting on it will be drag from the air. My guess that the higher the rpms, the faster you will lose rpms.
Third question:
Really tough. If the rpms have slowed down to exactly 1/2 what they were at the muzzle, then the answer is yes.
Lets see if I can cypher this out.
Time for 1 revolution will be 1/ ((rev/min) *1min /60s)
Distance for 1 rev will be velocity f/s *12in/1ft * time for 1 revolution
So Velocity in f/s * 12 in/ft *1/ ( rpm in rev/min * 1 min/60s)
Simplify (V in ft/s * 12 in/ft ) * 60s /rpm in rev/min
V at distance*720 /rpm at distance will give you how many inches it takes to complete one revolution. The rub is trying to measure the rpm at that distance.
Some one check my math
Hey Forearmed, this is classic physics stuff. Care to comment?
7br aka Mark B.
On the internet, I am 6ft tall, good looking and can dance.
Oh I hate math!
But the question is--what should the spin be at 500 or 1000 yards for stability, compared to the velocity? What is the starting spin for each velocity? What is best for 100 or 200? What if boolit length is changed?
I have to side with Bullshop and might need a shorter boolit in my Browning. So many BPCR shooters tout long, heavy boolits but that might be wrong for the twist rates.
44man
Boy did I ever misread what you saidDefinately my bad and please accept my appology. We both indeed said the same thing.
Bullshop definatel that is also the case. We see many examples of that at shorter range with handgun loads that shoot well at 50 yards but poorly at 100 yards. Same with low end .22 Hornet loads with a 16" twist. Mine will shoot 225415s like a house on fire at 50 yards but it groups improved cylinder at 100 yards if velocities are under 1800 fps.
It is very noticeable with pellets in pellet rifles as we extend the range. At some point the pellet just loses it and goes helter skelter.
Larry Gibson
I have had the problem of trying to get a replica spencer 56-50 to hit any thing at100yds, the the twist rate is 1 in 20 (way to fast for a .521cal .775 long bullet,that is needed to cycle properly in the Spencer)I can get very good 100yd acuracy with a .975 long bullet but because the OAL can not exceed 1.61 to work the pouder charge has to be reduced to 28gr FF black,what I am after is a twist rate that will be acurate from 50 to 300yds with a velicity of around 1000fps yousing the shorter bullet.can any one work out what would be the best twist rate for this that would be stable over this distance given the volicity and bullet lenth problems
so in order for the rpm's to be accurate at longer distances did it need to be over stabilized at shorter distances ? would that not be considered" going to sleep "?
7BR,
Your math is a worthy and valiant effort, and is to be commended. The rub inherent in the question, "What is the length of travel for one revolution at N distance" is that the question involves much more complicated math. One must calulate the amount of spin damping that the bullet undergoes, as this affects the rate the bullet rotates at, and thus the distance over which it makes one full rotation. The rate of spin (ros) is constantly slowing from it's initial rate, and the ros at distance is dependent on the rate of spin damping. The rate of spin damping predicates on the atmospheric conditions, the surface geometry of the bullet, velocity, and the relationship of inertial dynamics to fluid flow over the bullet.
What most shooters do is only worry about initial rpm at the muzzle, same as most shooters only measure velocity near the muzzle.... and then test for performance at target.
Regards, and good shooting.
Yes, that's the way I take it. Like I explained, actually watching bullets showed some very strange things but it can't be done with high power rifles, etc. Darn boolits need to be large enough and slow enough to see.
Some say they see a vapor trail from rifle bullets like from a fighter going through the sound barrier but you can see trails from much slower plane wings too. Might depend on the moisture in the air.
Then can it also be the bullet has not yet gone to sleep and is spiraling? That would disrupt the air more.
The TC 30-30 I talked about was a strange puppy with the boolit going sideways and returning to the original aim point. The gun was deadly accurate at 100 yards and with a scope from sandbags I have a pile of nickels and pennies with holes in them. One day I ran out of gas checks so I left them off. Every single boolit would go through the paper sideways at 50 yards. The only thing I can figure is I changed the bearing surface, making the boolit shorter and no longer matching the twist for the velocity.
I would say I was shooting the boolit at the ragged edge of stability by too high a velocity and too much spin, yet it always hit what I aimed at taking into account the windage changes at different distances. Did the length of a gas check really take it out of stability? Would reducing velocity have fixed it so I would not need the GC? My friend wanted the gun so I sold it before finding out.
Anyway you see why I can't recommend twist rates for anything. Too many tiny things can change what works and even a 1/2" change in twist rate might go one way or the other.
I only like a rate that works in the velocity range of a caliber.
My thinking is always mechanical because at my age I can't remember how to add 2+2!
The last few posts seem to 'spin' off some to my thinking....
Here's an interesting paragraph from Lilja Barrels... Daniel Lilja wrote it in fact. Lilja is pretty well known for ACCURATE barrels. I know many use them at long range...
"It is interesting to note that this force, or overturning moment, acting on the bullet nose is greatest when the bullet first exits the barrel. If a certain bullet-barrel combination will stabilize a bullet at the muzzle, the bullet will remain stable for the rest of its flight. Why is this so? As a bullet flies on toward the target and beyond, it is losing velocity rapidly as any trajectory table will show. For example a 68 grain bullet from a 6PPC leaving the barrel at 3150 fps will be going 2786 fps at 100 yards and 2449 fps at 200 yards (ballistic coefficient of .265 and standard metro conditions). The rotational speed of the bullet or its RPM's decreases at a much slower rate. The overall result is a lessening force acting on the bullet nose, and that force is being overcome by a proportionately greater spin rate compared to forward velocity. The down-range bullet therefore is more stable than it was at the muzzle. The only exception to this occurs when the bullet passes through the speed of sound. At that velocity, about 1180 fps, it may lose stability."
The full article can be seen here:
http://www.riflebarrels.com/articles...ling_twist.htm
I found this paragraph in another paper 'deep' and interesting as well...
"As the projectile flies downrange, it attains an “over-spun”condition. At launch, the flow near the rifling grooves is essentially parallel to the grooves as a result of the no-slip boundary condition as shown in Figure 1. However, as the projectile flies downrange, the projectile’s forward velocity slows faster than the projectile’s spin rate, such that the nondimensional spin rate (pD/V) increases. This causes the flow near the grooves to be no longer aligned with the grooves(shown in Figure 1), producing an “over-spun” state. It is suspected that the over-spun condition downrange results in a change in the aerodynamic characteristics of the projectile."
Full write-up here:
http://pdf.aiaa.org/preview/CDReadyM...V2006_6009.pdf
Worth pondering on in my opinion.
Eutectic
44man
Pitch and yaw are common in well stabilized bullets at close range. Many times they are induced by "over stabilization". When they "iron out" is what is refered to as "going to sleep".
There are varying degrees of stabilization as with any bullet there is a wide RPM range between minimally stabilized, stabilized and over stabilized. It is best to have the bullet in the "stabilized" range of RPM at the beginning of flight. Your example of the .44 bullets in the S&W (should have had an 18 3/8" twist) are a good example. At the velocity the lighter weight bullet was overstabilized and the heavier bullet was stabilized, both for the velocity they were at.
If we shoot cast bullets in the "stabilized range" the bullet should remain stabilized over the expected distance of flight. Thus stabilized most all bullets will remain stable long after the maximum range at which we shoot them is reached. If a bullet would go unstable from lack of velocity and/or too much RPM at 2000 yards does it matter if we only shoot 300 yards and the bullet has already been stopped in the berm? What we want for the best accuracy is the best stabilization across the range that we shoot at. What it might do beyond is moot.
In the case of 160 - 220 gr cast bullets in the '06 with 10" twist the optimum velocity for the "stabilized" range is 1600 - 1950 fps. If we have the same bullets in a 12" twist then the "stabilized" range moves to 1900 - 2300 fps. If you do the math the RPM is pretty much the same for both ranges.
Having the ability to actually measure the TOF and BC of a bullets flight over 100 yards gives us the measurement of a bullets stabilization. The less the TOF and the higher the BC the more stabilized the bullet is. I also have several .308W cal rifles with 9.5, 10, 11, 12 and 14" twists and several .223 rifles with 7, 9, 11, 12 and 14" twists. It is easy to measure a bullets stability, given a relatively equal velocity, out of those and determine which twist is best stabilizing that particular bullet.
Larry Gibson
I find this is the very biggest key to most shooting. It translates out to it's all in the launch.
Launch perfectly and muzzle pressure affects the launch uniformly (no yaw) and shooting variables are minimized to get a slug to travel through air accurately without the need to be spun faster. Which is why some guys / guns can achieve more.
Twist rate compensates for error or ignorance making experts of everyone right up until the point bullet material fails to launch well for what ever the reason.
Reading can provide limited education because only shooting provides YOUR answers as you tie everything together for THAT gun. The better the gun, the less you have to know / do & the more flexibility you have to achieve success.
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