/

Alf, you may want to verify this with the makers of the Rhino bullet, but the little diagram on the box of the Rhino Solid Shank bullet shows a copper jacket and shank with a lead bonded filler in the FRONT of the bullet. Your question # 3 has already been answered by Rhino bullets, at least I think it has.

Reread this and came back to edit. The description of the Rhino bullet doesn't really answer your question # 3. I'm just pointing out that the design you suggest as theoretically more stable does exist. But you knew that.

]

quote:

nor can a projectile be stabilized by spin in dense media due to the high density of the medium? Spin stabilization is in fact negated by the density of the target medium hence the propencity for all oblong projectiles to eventually tumble in target

Alf, I know you have problems with the vapor bubble surrounding the bullet in aqueous tissue, but this absence of denser media is utilized by the spin stabilisation, a little bit more than for stabilisation in air is recommended.

quote:

A further point in terms of the use of FN projectiles or cylinders is that the apparent mechanism of stabilization is due to shoulder stablization and this entity is not spin dependent ! Cylinders fired from smooth bore guns ( thus spin negated will remain stable in target).

Nobody has confirmed this idea for FN hunting bullets. Cylinders fired from smooth bore are definitely not stable in air and target.

]

quote:

Shoulder stabilization of cylindrical bodies works fine in dense media because of the very high forces.

Again, he gives no experimental proof. It is pure theory and not to maintain. In his newer publications he is going steps back, no longer claiming it for absolutely all cases.

?

Where's 500 grains when you need him..
AK

Although not germane to the big bore bullets, twist does have an effect on projectile function as the velocity goes up...expecially on the far side of 3000fps. Velocity drops at a variable rate, based on BC. Rotational decay does not. Imagine you have a 1:12 twist. A projectile going 2000fps from a 1:12" twist has a moderate spin rate. Take twist (in feet) times MV and multiply times 60 (seconds per minute) IE 1 x 2000 = 2000rpm x 60 seconds per minute = 120,000 Revolutions Per Second. Make that twist 1:9, and now it is going 1.33 times as fast. That bullet now is spinning at 159,600rps. The 9" twist is fairly popular for 6mm bullets. Say you have a 243 and you are doing 3000fps MV from that 9" twist. Now that bullet is traveling 239,400rps. For the punchline, draw a circle about 2" in diameter. Draw crosshairs centered left and right and up and down. That intersection is geometric center of the bullet. Put a dot just a teensy bit out in any direction. Due to production tolerances let us consider that is the actual center of gravity, and it is just .0001 (one ten-thousandth of an inch) from geometric center. As soon as that bullet exits the barrel those two centers are in conflict, and the bullet is trying to spin centrally...but goes from one center to the other about 239,400 times per second. A very small wobble ensues, and if the rotational torque is sufficient (due to fast twist or RPS or bullet weight) that bullet will burst from the centrifugal forces at work. That is why you hear stories about the 220 Swift "blowing" bullets up. Too much torque for frangible 22 caliber bullets. Cure: thicker and more uniform jackets.
Too much twist and you "over-spin" a bullet. It will yaw a while before settling down into a smooth flight.
Too little spin, and the bullet never stabilizes. Like spinning a top. Spin it too slow, it just falls over. Spin it too fast, and it dances all across the tabletop and spins off the edge.
Greenhill formula: Length in inches divided by diameter in 1/100ths of an inch.
Divide 125 by above number. Multiply step 2 by bullet diameter in 1/100ths of an inch. That is optimal twist rate.

Waaaaaaaaaaay more information than you needed, right?

Rich

IdahoSharpshooter,

You have your units screwed up in that last post!

Alf,

quote:

(In air a bullet must turn 3000 times per second to be stabilized!)

I can think of a good many rifle cartridges that will never ever get their bullets stabilized if that rotational speed is correct!!!

Just think about it. 1:10" twist at a muzzle velocity of 2250 fps. That is a rotational speed of 12/10 * 2250 = 2750 rev/sec. That isn't stable (explains why the 30-30 ain't accurate). How about the historic 416 Rigby with it's 1:16" twist. That gives us 12/16 * 2370 = 1778 rps(rounded up), now that is LOW!!
Finally, the 50 BMG, the cartridge and rifle our very own Robgunbuilder extolls, with a 1:15" twist at a muzzle velocity of 2750 fps what do we get; 12/15 * 2750 = 2200 rps, according to your expect not stablized. I am thinking that only small bore ultra high velocity bullets can therefore be stable in flight, and then only until their velocity drops below ~ 2900 fps (not too aweful far from the muzzle really).

My $0.02 worth. Carry on.

)

Yawohl, Rich switched his rps's with his rpm's.

Herr Professor Beat just pulled the 3000 rps figure out of his ass, but hey, 2000 rps or 3000 rps, no big deal, it is in the ball park.

If shoulder stabilization in dense media is real, it should also be of some effect in air, however it seems a short cylinder fired from a smoothbore would be more likely to tumble in air than a long cylinder, for reasons that overcome the shoulder stabilization. Certainly a nose heavy cylinder would steer straighter from a smoothbore. But concentricity issues with a heavy nose might defeat a real-life bullet from a rifled barrel.

If gyroscopic stabilization is present in air, it should also, at least in initial penetration of dense media, allow some tiny increment of improved stabilization, and every little bit helps, when the chips are down: In a hunting rifle, therefore, use more than enough twist than the benchrest-paper-punching idealist choice.

Just whistling in the dark, I am, enjoying the spectacle.

.

Alf's Q1:

quote:

How does barrel twist influence solid bullet penetration other than contributing to stable flight and thus a small angle of attack angle of incidence on impact?

quote:

Angle of attack (α, Greek letter alpha) is a term used in aerodynamics to describe the angle between the airfoil's chord line and the direction of airflow wind, effectively the direction in which the aircraft is currently moving. It can be described as the angle between where the wing is pointing and where it is going.

In ballistics we should correctly refer to this very important aspect in bullet impact with the target as the angle of incidence! (Afrikaans: invalshoek of hoek van inval.)

]

Question one:
"So where exactly does rate of twist come in to the equation when dealing with terminal behaviour?" If we disregard the gyroscopic stability requirements of long range shooting and consider only hunting at distances to 500m, more twist will result in higher gyroscopic stability in flight and aid in stable transition from air to tissue. That is it's value. Once the bullet has transitioned form air to tissue, another set of parameters kick in and there is little point in discussing twist rate from there on.

Question two:
The factor that tries to overturn a projectile in flight is the lever that runs from the centre of pressure to the centre of gravity of the bullet. The longer the lever, the easier it is to turn the bullet. The centre of pressure of a bullet with a lead front core is at the tip of the bullet. The centre of pressure of a hollow point bullet is down the hole somewhere. The lever is shorter with a hollow point and the bullet will dampen out the precession cyles faster than a lead tipped bullet. Less yaw better BC and so on. Give the bullet a boat tail and it shifts the cg forwards, further shortening the lever but adversely affecting the gyroscopiv stability by reducing the rotating mass at the circumference of the bullet. Six of one and half a dozen of the other.

Question three:
Any time a solid is constructed from more than one type of material, one will try to separate from the other on impact and the reliability of the bullet will suffer. It is better to have a reliable solid which performs in the same manner all the time than have performance highs and lows that are unpredictable. Murphy's law dictates that performance highs will occur when shooting dassies and targets and peformance lows will occur when you are smelling the lion's breath.

quote:

Due to production tolerances let us consider that is the actual center of gravity, and it is just .0001 (one ten-thousandth of an inch) from geometric center. As soon as that bullet exits the barrel those two centers are in conflict, and the bullet is trying to spin centrally...but goes from one center to the other about 239,400 times per second.

A problem that exists only with bimetal bullets and not with turned monometallic bullets.

quote:

Too much twist and you "over-spin" a bullet. It will yaw a while before settling down into a smooth flight.

I don't think this is quite correct. The higher the gyroscopic stability (higher spin rate) the quicker the fast and slow precession cycles are dampened. The coning motion of the bullet can be caused by bullet imbalance but is mostly due to a small angle of attack that exists as the bullet exits the bore.

quote:

The main apparent contribution that correct twist has on the penetration process seems to be related to angle of incidence and not angle of attack.

Angle of incidence is called Anstellwinkel in german and descibes the angle between the long axis of the bullet and line of motion thus yaw angle.

Angle of attack on the other hand is the angle between line of porjectile motion and target surface. Also known as Auftreffwinkel in German.

Alf it is the other way around. Angle of incidence is the angle at which the bullet meets the target as in a broadside shot or a quartering shot. Angle of attack can be the result of a bullet with too much gyroscopic stability and occurs in the falling section of the trajectory or it can be the result of the yaw of an unstable bullet.

Gerard,
Thanks for the clarification on angles of attack and incidence.

The angle of incidence is surely more important in discussing armor plate penetration than game animals.

Angle of attack (deviation of long axis of bullet from line of trajectory) is indeed all important in any situation.

So from 0 yards to 400 yards (a hunting rifle's practical application), more twist (than the ideal 1000 yard paper punching twist) is never a problem.

The quicker twists allow no worries with the longer for weight monometals.

If I want a 1000 yard rifle, I would indeed use the slower "ideal" paper-puncher twist, matching the length of bullet and consider velocity too.

But ... Geez! 239,400 times per second! Be careful in quoting. That is way off, a units screw up by Rich, who needs to correct that. Though I know you were more interested in another aspect of the quote ...

RIP,
Angle of attack on impact spells bad news the majority of times and is worse with bimetal bullets, that are more prone to breaking up than monos, when subjected to impact stresses when traveling sideways.

Angles of incidence cause asymetrical deformation of the bullet. If the asymmetry is severe enough, the bullet will deviate from the line of flight inside the target. It causes a bullet to exit on the same side as the entrance hole in cases of extreme angled shots. Slow twist rates are more likely to do this than fast twist rates. I have seen this three times under field conditions (150gr lead core bullet from a 270 Win with 1:10" twist, twice with 243 Win 1:10" twist with 100 / 105gr lead core bullets) and many times in testing on various media. Again, asymetrical deformation occurs more frequently and to a much larger degree with cup and core bullets than with monos.

The monos below still performed acceptably while bimetal bullets, under the same conditions, fail more frequently.

Gerard,
I have had such a sheltered life that I have never witnessed bullets coming back out the entrance side, but I will take your word for it. Glancing shots into the extreme quartering sides I try to avoid.

I'll be looking for a delivery of GSC HV's soon, eh? Sure hope they beat Christmas.

RIP,
I have been fortunate to have witnessed the hunting of a very large number of animals. Further I have seen the results of hunting an even greater number of animals within a day or two after they have been shot and been able to talk to the shooters. Lastly, I have personally hunted a fair number of animals with a wide variety of calibers and bullet types.

I will let you know as soon as SA Post hands off to USPS. It should be any day now and then it is a short hop to you.

Alf,

Would you please post .pdf's of each of the references that you cite? Thank you.

RIP,

A bullet exiting the same side it went in on is an occasional phenomena with Barnes X of traditional weight (such as 400 gr. 416) due to instability.

Example 1: Buffalo shot head-on front of the chest with 300 gr. .375 H&H Barnes X. Bullet enters skin, curving around the side of the chest, travelling just under the skin down the length of the buffalo to rest under the skin just ahead of the hind quarter. Bullet never entered the chest cavity. To follow this path, the bullet had to move more than a foot off to the right of the point of impact.

Example 2: Buffalo shot perfectly broadside with Barnes X 400grain .416. Animal is not found until the end of the next day. Little blood. Bullet hit perfectly behind the shoulder 1/3 of the way up from the bottom of the chest. Bullet did not enter the chest cavity. Instead, it curved upward around the outer periphery of the rib cage under the skin and came to rest in meat next to the spine. In order to get anything like that kind of bullet path with straight line penetation, the shooter would have had to have been lying in a pit beneath the buffalo shooting straight up. So in this case the bullet changed direction 90 degrees immediately upon hitting the animal.

500grains,
Noted. Thanks for the further examples, we add to our anecdotal list.
I still think a .375/300-grain monometal copper soft or solid is O.K. at the higher velocities of the .375 UAE or .375 WBY or .375 LRO: 2750 fps. Saeed has certainly proven this beyond a doubt.

Agree that the slower-for-bullet-weight .375 H&H ought to be limited to 270 grains with the monometal copper bullets in the same 12" twist.

.416's ought to to stick to 380-grains or less.

All would be better hunting rifles with 10" twists.

]

?

I believe that a spin stabilized projectile is better served by the CG being toward the rear.....

It seems to be a matter of semantics and you best get on the same track as the author.

Using "angle of incidence" by Alf's system would require switching the meanings of angle of attack and angle of incidence to the opposite of Gerard's.

It is a convention.
I can accept the usage either way, but it ought to be consistent. But it obviously is not.

I must say Gerard's nomenclature seems more logical:

Angle of incidence referring to the path (line of trajectory) of the bullet center of gravity relative to the surface impacted (flat or tangent to point of impact), and angle of attack referring to the yaw of the bullet from the line of trajectory.

Just my warped mind?

I was at Olin corporation a few years ago and they have a computer program that helps an engineer design bullets.

It shows you the CG of the bullet as you increase or decrease jacket thickness, change from gilding metal to pure copper alloy, change from lead to tin-lead, and shows CG change as you move a partition forward or back. Same with a solid base (TBBC design).

It is not hard these days to move the CG wherever you want.

The Speer Grand Slam is a good example of what Alf is talking about. For many years they advertised that the soft lead was in front of the hard lead rear core. But infact they sold quite a different bullet with the soft (heavier) lead in the rear and lighter (harder lead) in front so they would be more accurate!

I was able to design a number of experimental bullets just for fun while at Olin. Modern software makes bullet design relatively easy.

I thought I had made a terrific big game bullet with solid copper ogive and bonded lead rear core, only to find out that it was highly likely to turn over when it decelerated. (The ogive was too light and CG way too far to the rear).

Back to the drawing board! But with their software, this only took a few minutes.

(I had basically re-created a TB Sledgehammer).

I got hung up on a hard nose without thinking about the effect on CG. This would have made a decent anti-personnel round, but a lousy elephant load!

For stability, they recommended a CG that coincided with the bullets center of form. Designers of military bullets at FN Herstal and Aberdeen were certainly aware of this, but they tried to de-stabilize their bullets (unless they were inteneded as AP projectiles in which case stability was once again very important).

The trick w the Olin software was to design a bullet that was still well centered after it expands. This is one reason they re-inforced the Fail Safe bullets lead core with the steel sleeve and base disc to prevent the core from shifting as the Swift's rear core does.

Andy

.

Interesting topic, but man there's alot of mixed up terminology here! Gyroscopic precession, is the "wobble" exhibited by a bullet with a slightly off of centerline CG. Center of pressure can also be an aerodynamic term and it does affect a bullet in flight. Yaw is positive or negative motion along the horizontal, pitch is the same along the vertical axis. How about a glossary of terms next time?

I love reading these discussions but boy does my head hurt Keep it up

Alf,

Since you are interested in military projectiles, I thought I should point out that the diagrams of bullet profiles that Janzon published appears to be incorrect for the SS109/M855 and the 5.45 x 39.

This may simply be an artifact of the loss of resolution between the original, a photo scan and posting at 72 dpi, but check your original for the following:

1. The M855 differs from original SS109 in that the 10 grain C70 Rockwell hardened steel penetrator in the ogive DOES NOT protrude into the interior core of the bearing surface as this picture suggests. The original SS109 did, just as your diagram shows, so he undoubtedly had some original SS109 to base this on. Tests with the M249 (MINIMI) LMG demonstrated that this tiny intrusion into the bearing surface reduced barrel life from about 25,000 rounds to less than 10,000 rounds. (It is VERY hard steel).

The M855 added a bit of lead to the core to raise the penetrator above the begining of the ogive.

2. The 5.45mm core weighs 25 grains and is mild steel, about B50 Rockwell and has two boat tails, so it can be inserted either way in the high speed swaging machines. This diagram makes it look like it is a boat tail on one side only (and a wadcutter ont he other). The truncated cone core has better penetration as a TCFN than a wadcutter anyway.

Very small details, but thought you would like to know.

Andy

RIP wrote:

quote:

I must say Gerard's nomenclature seems more logical:

Angle of incidence referring to the path (line of trajectory) of the bullet center of gravity relative to the surface impacted (flat or tangent to point of impact), and angle of attack referring to the yaw of the bullet from the line of trajectory.

Agree -

quote:

Angle of attack (α, Greek letter alpha) is a term used in aerodynamics to describe the angle between the airfoil's chord line and the direction of airflow wind, effectively the direction in which the aircraft is currently moving. It can be described as the angle between where the wing is pointing and where it is going.

On this page you will find:

angle of attack = Anstellwinkel = Angriffswinkel
angle of incident = Auftreffwinkel

The Figure 4.4.4 you show above, illustrates a bullet with an angle of attack approaching a target at an angle of incidence and is therefore correctly captioned as angle of incidence or Auftreffwinkel. The impact angle is the angle of the bullet axis to the target when it strikes at an angle of incidence and having an angle of attack. There are three concepts (of angle) contained in this illustration.


The Figure 6.31 is also correct as it shows the Anstellwinkel (angle of attack) that is formed when the bullet axis differs from the direction of travel.


This is the same explanation as in my illustration below.

Re: shoulder stabilisation
Alf, my list of publications is only about half of that from B. Kneubuehl, but there are some discrepancies to comment:
you wrote:

quote:

I did not say or infer cilinders are stable in air without spin.

K.: Geschosse, Band 2: Bullets for smooth bore: Stabilisation in air is possible with a big, plane front area. This effect is called "shoulder stabilisation". No spin needed.
K. quoted in your post:

quote:

Shoulder stabilization of cylindrical bodies works fine in dense media because of the very high forces.

He argues only for very short objects, the CG exactly in the center. In a graph the lenght is given 1.2 times the diameter. Here shoulder stabilisation can work, I remember this is used in some spacecraft and shaped charge devices.
But:

quote:

A shaped projectile begins to turn in dense medium because of the big force on the tip and a small angle between projectile axis and direction of flight.

Hunting bullets are shaped projectiles. He writes: Only the nose of the bullet is in contact with the medium (benetzen). The flow of the medium cuts off (Strömung löst sich ab).
Not saying, what is between the bullet and the flow, but very similar to my "vapour bubble".
In Band 2 he writes: Water is a very suitable simulans because of the high water content of tissue.

If somebody still is convinced that shoulder stabilisation works on FN hunting bullets, we may assume:
-RN solids with a semispherical nose (old Barnes or even Woodleighs) are absolutely unstable and should not used on game.
-the angle of incidence should be 90°, otherwise the bullet is turning in an erratic direction.
-a stable flight in inhomogeneous tissue is impossible. Any variation in density would cause veering off.
-test shooting through baffles of particle boards, about an inch separated and under 45° to the bullet path should show an amazing bent curve.

.

i'll still stick with my woodleigh solids despite all these theoretics, they work !

quote:

Originally posted by tomo577:
i'll still stick with my woodleigh solids despite all these theoretics, they work !

Yep! tomo577 got it exactly right. Why change from what works!

465H&H

quote:

i'll still stick with my woodleigh solids despite all these theoretics, they work !

Do not lose sight of the fact that these theoretics are surfacing to try and find reasons why FN style bullets and monos work better than old style bullets. It is not about old style bullets that have stopped doing what they do, but about trying to explain the mechanism behind an advance in technology.

Norbert, something else to recall and remember:

Alf wrote:

quote:

Soft solids do not do this, tissues do not do this.
They leave us with cavities filled with damaged dead cells and srtucture!

My conclusion of months ago then:

Só, because of the above Alf wrote now, I maintained many months ago that the Bernoulli principle of "stagnation pressure" can never, ever be applicable in terminal ballistics.

Secondly, also because all the above in terminal ballistics doesn't take place in an enclosed pipe-environment as required by Bernoulli's whole theory.


The real reasons behind a bullet's final mushroom shape.