quote:

I can find no forces to cause this to happen.

Gravity.

quote:

Originally posted by DigitalDan:

quote:

I can find no forces to cause this to happen.

Gravity.

Yes...it's being pointed out nicely.....and Hot Cores statement is directly in line with the comments I've heard from the 1,000 yard shooters.

quote:

Originally posted by Reloader:
I would tend to think that as the external Forces contact the bullet that the 1:10 and so forth get thrown out the window. Especially w/ fast velocity, the only time it will spin a set amount of times is when it's in the barrel. I would imagine the spin slows down from the data I've seen. What do you fellas think?

Reloader

"OK, I think I'm catching on. It's rotational spin will basically remain constant once set in motion, no matter what its forward movement. Am I on the right track? I would guess that at 150 ft and the "several more rotations" would be minimal.

Yes, you are on the right track. After being set in motion, if a spinning bullet could be set on its' nose on an immovable object that stopped its' forward motion, it would sit there and spin like a top until it finally lost all rotational monmentum and toppled over.

Spin does slow down, but at a statistically insignificant rate, as there is very little friction acting on the bullet's rotational momentum until it hits something.

quote:

When you see folks talk about l-o-n-g distance shooting and they start mentioning "Clicks of Elevation", if they do not also mention "Clicks of Windage", you can make a relatively safe bet they have never fired a shot at distance.

Thanks - good info Hot core,
Do you have any idea how many clicks is common on say a 1000yd target. I have never shot 1Kyds

Stepping into what seems to be an on track discussion, hopefully to clarify something...or maybe not.

The issue of bullets nosing over as they begin their descent from max ordinate is a characteristic called tractibility. At the risk of starting a firestorm I'll say that all bullets of proper design, and properly stabilized exhibit this characteristic, and will in fact land nose first in all reasonable circumstances, including very high angles(in excess of 60*) of fire. I am aware that there are conflicting results on this from different sources and circumstance, it does not change what I stated above.

Gyroscopic stability is a funtion of angular momentum overcoming the overturning moment in conical projectiles caused by the center of pressure being forward of CG. Larger diameter projectiles generally required less rotational velocity to overcome the overturning moment, hence the slower twist rates usually associated with them. This is simply due to having mass at a greater distance from the axis of rotation than smaller projectiles.

Bullets are exposed to a great number of external forces in their flight, drag, imbalance, wind, gravity, and the influence of gyroscopic forces imparted by the rifling. Oh, there is the occasional gnat as well. What Hot Core referred to regarding bullets drifting to the right with a right hand twist(yes, they go left with a left hand twist) is called Spin Drift, and is a result of deflection caused by an offset of the bullet's rotational axis due to gyroscopic precession. The cause of that is gravity. Bullets with high gyroscopic stability factors(Sg) will do this with more pronounced effect than those with lower Sg. Now I'm going to walk slowly away from this subject and move to the next. Keep your hands where I can see them.

The question earlier regarding rpm decay vs velocity decay....as most have pointed out the influence of drag is much much greater on velocity than spin. Warren Jenson of Lost River Ballistics posted once at LRH on the subject and proferred a formula IIRC...from memory the amount of rpm decay on his .408 Chey-Tac at 1500 yards was about 5% or less. I do not recall the formula...it ain't worth worrying about. Look at it this way. If a deer has a 12" wide chest and you're shooting a 1:12 twist AND the bullet does not decelerate for some mystical reason, you get one turn of the bullet as it passes thru the deer's chest...at 100 yards or 600 yards. In the real world the energy stored as angular momentum in a bullet is VERY small, and when the bullet expands the rotation slows or stops very quickly. So much for the 'Buzz-saw' theory. Real world deceleration forces on a bullet in flight are much more severe, realistically measured in terms of 40-60 G's to give it understandable quantification. Those drag forces are variable according to a bullet's BC during flight.

Anyhoo, there's a whole bunch more info out there if you're interested. Exterior ballistics is a pretty deep subject and good exercise for the mind. Assorted works by Vaughn, Pejsa, McCoy and others can keep you drooling into the wee hours, and their knowledge is there for the looking. Don't go there if you're looking for a casual read.

Good discussion by the way, keep on keepin' on.

Happy 4th!

quote:

The issue of bullets nosing over as they begin their descent from max ordinate is a characteristic called tractibility. At the risk of starting a firestorm I'll say that all bullets of proper design, and properly stabilized exhibit this characteristic, and will in fact land nose first in all reasonable circumstances, including very high angles(in excess of 60*) of fire. I am aware that there are conflicting results on this from different sources and circumstance, it does not change what I stated above.

Including a bullet fired vertically up (90 deg)???

BTW...good read Dan...it's good to have a discussion that is off the beaten path...

Depends on where the CG is. The back of the bullet will have flat plate drag and based on this drag and it's rotation it will most likely fall "off" to one side at which point the disimilarity in drag flat base to rounded nose will cause a further turn "into the wind" so to speak. All things being optimal for the aerodynamics of course. I would venture to say a bullet fired even at 85* vertical inclination would land point first as the path is not in a single plane, it is a parabolic arc a steep one but an arc none the less.

Vapo, I think Planemech is pretty much spot on with that. I'm not going to swear in court that a particular bullet on a particular angle of elevation will do X, Y or Z, but there is information in Robert McCoy's book "Modern Exterior Ballistics" that stipulates tractibility of 105mm howitzer shells at 85*. They wobble a bit then nose over, and down they go, pointy end first.

BTW, terminal velocity of bullets is quite a bit faster than 120 mph. I don't recall who threw that out, but speeds of 600-800 fps are more common for small arms bullets. I'd guess arty shells do a good bit more given their high BC's.

quote:

BTW, terminal velocity of bullets is quite a bit faster than 120 mph. I don't recall who threw that out, but speeds of 600-800 fps are more common for small arms bullets. I'd guess arty shells do a good bit more given their high BC's.

That was my number...something I read years ago...and also based on actually being hit on the head with falling lead BBs duckhunting. They don't even sting!!!! purely a guess to make another point.

Look folks...you've done well explaining the forces that cause a bullet to drift left and right with LH and RH twists.....and the forces that can cause a bullet to turn when fired at such an angle that air resistance hit's the profile of the bullet.....but I'm not at all seing any forces that can cause the gyroscopic effect of several hundred thousand RPMs of a bullet fired 90 Deg vertical to turn over....this one defies my logic cells..... I see a bullet somewhere a few miles in the sky at 0.0 feet/sec and starting to fall to earth but still spinning somewhere to the tune of 100,000 RPM....this thing isn't going to turn around for anything unless it's acted upon.....

Maybe it's all them physics and math courses that have my thinking screwed up!!!!

Lets try again.....what forces cause a bullet fired vertically to turn over given that it's working like a gyroscope?

if you guys were closer, I'd take you out for a beer.....Linies anyone?

quote:

Originally posted by jeh7mmmag:
...Do you have any idea how many clicks is common on say a 1000yd target. I have never shot 1Kyds

Hey Jeh, You can see the effects of it much closer than 1000yds.

No, it is not possible to give you Windage Clicks that would have any significance. If you just start with the Scope and consider it, not every scope has the same amount of adjustment per click.

Different twist rates in the barrels, different velocities, totally different bullets, and when they make Exit from the Bore, the surface condition of the bullets are different from rifle to rifle.

If you had twenty rifles as identical as possible shooting the exact same Load, you still might not get one shooting the same as another one due to all the above reasons, plus slight differences in the way the Barrel responds to the Vibrational Harmonic.

The only way to know what it will take for a specific Rifle/Load is to actually shoot the rifle at distance and then add it to the old Drop Chart.

In fact, the difference in B.C. from rifle to rifle can also have a significant difference on the Clicks of Elevation.
---

If you ever talk to a guy who is "bragging" about Long Distance shooting, just ask if he ever has to put in any Windage. Don't even mention "Left" Windage.

People that can - don't brag, people that can't - usually do.
---

quote:

Originally posted by DigitalDan:
...Now I'm going to walk slowly away from this subject and move to the next. Keep your hands where I can see them. ...

---

quote:

Originally posted by VapoDog:
...if you guys were closer, I'd take you out for a beer.....Linies anyone? ...

Good gosh, a man from Wisconsin that doesn't appreciate the World's Finest Beer - that made Milwaukee famous - Pabst Blue Ribbon.

George Dickle and ginger in a rock glass, light on the Ginger please.

Vapo, for the sake of a theoretical discussion I'll give you my take on that. Opinion only, unsupported by learned wizards of ballistic science. So far as I know.

Given your parameters, I'll add a couple of my own. 90* elevation, perfectly balanced bullet, no atmosphere, no barrel vibrations...nada, zippo, zero influences to upset the apple cart. Given those conditions I think it likely the bullet would strike you base first in the muzzle of your rifle...assuming of course the celestial body from which the shot was made did not rotate itself, such as the moon. Ya can't forget coriolis force doncha know. Credit is given for the effects of gravity being perfectly parallel to the axis of rotation by the way.

In the real world none of those conditions could be met, so something is going to apply force, lateral force to the bullet, and that will cause the gyroscopically stabilized projectile to precess at right angles to the axis of rotation, perpendicular to the applied force. All bullets are unbalanced. All bullets leave the barrel with at least two modes of precession, a fast and slow cycle nutation called an epicyclic precession. In other words, they may be coming out of the barrel straight, but they don't stay that way. In fact they start getting the wobbles before the bullet is fully clear of the barrel. Accuracy is all about having consistant precession cycles and the bullet leaving the muzzle at the same point in the sine wave of barrel vibration...not perfection.

So, I'll buy you a round for posing the question for which there is no firm answer if you'll buy me one for rainin' on your parade. Mebbe it'll land on its butt, mebbe not. Don't forget the gnat factor....

Dan

quote:

Good gosh, a man from Wisconsin that doesn't appreciate the World's Finest Beer - that made Milwaukee famous - Pabst Blue Ribbon. beer

I believe it was Schlitz that made Milwaukee Fmous!!!

PBR is considered roughly equivalent to bullheads in a walleye world~!!!!!!!

DD, HC, planemech, jeh7mmmag, Frank, eldeguello and others....you guys put on a fine thread....maybe we can find another topic to stimulate the thought process...

I have a question about gravity, but it has probably noting to do with fireams and so I will not hi-jack my own tread.

But if I was to ask the question, it would be is gravity constant? Or is it always the same force? Has it always been the same force since our solar system was?

But I won't ask.

In any case the replies were very exotic and interesting and unexpected.

quote:

Originally posted by Exit31:
I have a question about gravity, but it has probably noting to do with fireams and so I will not hi-jack my own tread.

But if I was to ask the question, it would be is gravity constant? Or is it always the same force? Has it always been the same force since our solar system was?

But I won't ask.

In any case the replies were very exotic and interesting and unexpected.

Start a thread in Miscellaneous.....maybe some folks will pipe in there.

They certainly will. While you're at it, ask if your pickup is going light speed and you turn on the headlamps, will you see the light?

Nice discussion. I really enjoy the way most AR posters can get along and share ideas. This thread is a prime example of what forums should be all about! Salud, all!

As to the constancy of gravity... YES, it has been and always will be the same until the mass of the earth changes significantly. (Based upon the equation for gravitational force between two bodies as it is understood today - all bets are off if they ever find that Einstein was right and there are gravity waves, etc....)

Force of gravity between two objects solely depends on the masses of each object (bullet and earth, for example) and the distance between their centers.

quote:

Originally posted by DigitalDan:
George Dickle and ginger in a rock glass, light on the Ginger please. ...

You talkin' `bout Ginger from Gilligan's Island?

quote:

Originally posted by vapodog:
...I believe it was Schlitz that made Milwaukee Fmous!!!

I must have had toooooo many PBRs to remember correctly.

quote:

PBR is considered roughly equivalent to bullheads in a walleye world~!!!!!!!

Now there is a REAL endorsement.

quote:

Originally posted by 1 Shot Hunter:
... I really enjoy the way most AR posters can get along and share ideas. This thread is a prime example of what forums should be all about!

Yes indeed, never a disparaging word here.

quote:

As to the constancy of gravity... YES, it has been and always will be the same until the mass of the earth changes significantly.

Force of gravity between two objects solely depends on the masses of each object and the distance between their centers.

I do know "gravity" has had less of an effect "on me" since 15Mar06. Do you really think the earth's mass is changing?

,

quote:

Yes indeed, never a disparaging word here.

Well said!
I will second that. I lurk a lot and find the guys here the most intelligent of all Reloading forum
James

quote:

Originally posted by Hot Core:

quote:

Originally posted by 1 Shot Hunter:
... I really enjoy the way most AR posters can get along and share ideas. This thread is a prime example of what forums should be all about!

Yes indeed, never a disparaging word here.

Should I see if we can get Denton back?????

quote:

I do know "gravity" has had less of an effect "on me" since 15Mar06. Do you really think the earth's mass is changing?

HC, I do not lie awake worrying about such things! If the Black Hole theory holds any water it will...

15Mar06 - Gastric bypass surgery??? Just kidding, hope it was something good.

quote:

Digitaldan:

Maybe you can illuminate for me:

When the density of the medium in which the spin stabilized projectile moves changes ( becomes very dense or as when it impacts and enters a target ) how does stability and or the CP position change ?

So in effect air density change how does that effect stability. How much spin do you need at sealevel vs a projectile at very high altitude?

And if so why does a pointed or round nosed projectile now become unstable when the density gets very high ? At this point why cant stability be maintained?

At the same time why then does a cilindrical projectile remain stable in this situation?

Alf, I read your remarks in the thread about bang flops and it is apparent you know a lot more about terminal ballistics than I do. I agreed with most of what I understood you to say on the subject but admit I didn't follow all of it. Have the same problem with my wifey now and then, not that I'm implying anything by that. I will give a semi-literate opinion on your questions that may be as wrong as a football bat.

There is a linear relationship between density of the medium and overturning moment. In air it is well established that marginal Sg may well work at higher altitudes and not work at all at sea level. BR shooters can vouch for that.

When a bullet enters flesh the density is sufficiently high that Sg becomes meaningless in my opinion. A bullet travels a straight path either by random chance or due to bullet design. That is my take on why RN solids are less likely to deviate off course/tumble, and the same holds true for wide meplat bullets to include what I think you mean by cylindrical designs, or something akin to a DEWC. In the case of the latter the CG and CP are co-located and that cannot but help avoid deviation...assuming the bullet does not upset. In either case(RN/DEWC) the spread between CP/CG is small to non-existant, and they require relatively slow twists to stabilize.

There are of course many variables in the scenario and my thoughts on this are of the 'lab environment' sort. Dunno if you feel illuminated or not...not even sure I do, but those are my thoughts. Like I said, I don't know a lot about Terminal Ballistics.

If you would care to shine some light on the matter I'd be glad to see it.

.

Alf, you pose some interesting questions there, and at best I can only conjecture a little.

I cannot argue the twist/penetration issue either way. On the face of it I cannot imagine a substantive difference based on twist that could not be explained by statistical background clutter.

Your thoughts on change of CP due to fluid dynamics is beyond my knowledge though it seems you are addressing it in the context of cavitation? If correct I'd assume that largely a matter of impact velocity and frontal area. To, from my limited understanding of cavitation in fluids I'm not certain that the tip of the bullet is actually wetted. In any case, I don't know enough about that to comment intelligently. I do accept without question that the magnitude of overturning moments is proportional to medium density all else being equal, and that is the reason I don't think Sg has doodle to do with what occurs in the terminal phase.

Beyond H. Powley's computer I know little of his work, and I do not subscribe to the 'knock down' theory. I do believe sincerely that RN and the DEWC form or something similar is vastly superior in penetration, but I attribute that to nothing more than form, and perhaps cavitation factors, but not sure of that. Give a bullet an Sg of 1.8 in ISA conditions, then increase the overturning moment 900 fold....I don't see any other way to call it. Form.

Have a friend that shoots paper patched Hindenbergs of 510 grains from a .45-70, 1:30 alloy IIRC. When he manages to find one in the soft loam of the berm it often looks like a badmitton birdie...or a shot glass. Bullet base is plainly visible in the recesses but the darn things are inside out. I'm pretty sure than one is not going to stray far from the course in soft tissue, and that's about all I'm sure of. Scary ain't it?

Alf...If I may offer another opinion.....the world is full of questions without answers........and that's why I like to read actual field results.....see recovered bullets...read the accounts of others as I certainly can't have enough experience in my lifetime to say for sure about anything that isn't provable by math or physics.

You, yourself, probably have more experience with this than the next ten folks I know....While I really think I understand the question it's my premise that the best person to offer an opinion is you!!!

quote:

Originally posted by vapodog:
...Should I see if we can get Denton back?????

alf fills in right well for denton!

quote:

Originally posted by 1 Shot Hunter:
..hope it was something good.

Thank you!

Actually, very good - went on a slightly-modified Ethiopian Diet Plan(starvation). As the old DIs used to say, "Mind over matter...(etc)"

Without the use of a Chronograph(which ONLY tells Velocity anyhow), I have been able to determine significantly "Less Pressure Indications" from the Gravity Field.

quote:

Originally posted by ALF:
To my way of reasoning and maybe Im way off bat here the CG still remains in the same place within the bullet.

What happens to the CP?

My take is the CG (until deformation occurs of course) obviously stays the same. The CP might move a smidgen forward, but probably not that much. Talking about flat noses and round noses, the vast majority of the drag is coming from the very tip through air....

What has changed is the force at the CP. Any misalignment between the CP and CG causes an overturning moment...when the force at the CP goes up exponentially, this moment does as well. So if there is any hint of yaw, the bullet may fly fine through air, but eventually tumble in a critter.

Please don't quote me on that, as I just pulled it all out of my ass....

quote:

Talking about flat noses and round noses, the vast majority of the drag is coming from the very tip through air....

Help me, guys... I was taught the opposite - hence boat tail bullets with their superior BC.

Has fluid mechanics changed that much since I studied it??? What did I miss?

/

Yeah, boattails help a little, but they're a smaller driver than the tip. A sharp spitzer flatbase will kill the BC of a boattail round nose. But when looking for everything you can get, every little bit matters--it all adds up.

quote:

Originally posted by vapodog:

quote:

I don't remember what it is called (pre something) but there is a 3rd reaction when gyro is turned that would also make it vere of course.

Maybe someone can explain this....I can find no forces to cause this to happen. Rotation has no component in left right or any direction. What am I missing here.

I've heard people say that a bullet will drift right or left depending on whether the twist is LH or RH.....I'm not so sure but it comes from folks of credibility so I ask myself.....what am I missing here???

Find a serious book on Ballistics and look up "Gyroscopic Spin Drift". A bullet will drift slightly higher or lower in a crosswind due to the directional spin imparted by rifling. This is due to the reaction force imparted to a rotating object is Perpendicular to the imparted force. But don't take my word for it, look it up.
You might also find it in a gunners manual for helicopter door gunners, they run into this effect..................DJ

quote:

Rotation has no component in left right or any direction. What am I missing here.

Yes it does, according to the Right Hand Rule. Using your RIGHT hand, curl your fingers in the direction of twist. Your extended thumb will indicate the rotation vector. (With a right-hand twist, your fingers will wrap up on the left side of the barrel, thumb pointing toward the muzzle. Opposite for a left-hand twist.)

Then you "cross" the rotation vector with the wind, or gravity, or coriolis (earth's rotation) vector using the Right Hand Rule again, to see what is the resultant force vector acting on the bullet.

(BTW... Thanks guys, but I'm waiting for more responses on the BC issue - I'm not convinced yet! )

Go HERE , which is similar to most computer simulations to predict BC, type in the dimensions of a bullet and see where most of the drag is coming from. The base is only a small portion, easily outweighed by meplat/ogive drag.

Jon -

I'm interested in trying to verify some of these online calculators. It will be interesting to measure a couple of "known" bullets and see if this calculator correlates with the published BC's...

Obviously that will take quite a bit of work and a little time, which is precious at the moment.

Do you know if any studies have been done (that I can link to) where a boat tail bullet was shot backwards to test the BC? I have always thought the BC would be fantastically higher, backwards... But if indeed most drag is caused by the meplat, that would be clearly shown, too!

Thanks for your patience with me, and I hope I'm not the only one who has something to learn from this!

I'm not aware of anybody trying that particular test.... The better question is why would you think this is the case? Sure the base plays a part, but a bigger part than the primary shock wave? Seems really counter intuitive to me:



One thing you can do is look at the BC's of various bullet shapes. For example, a Sierra (who actually test their BC's) 180 Round Nose has a .24 BC at top velocity. Their Flat Based Spitzer has a .407 BC, an increase of 70% due only to the change at the front of the bullet. Adding a boattail to that gets it up to .501, only a 25% increase. A nice increase, but nothing nearly as big as the first. Do enough comparisons like that with various bullets and you'll see a trend....

quote:

Seems really counter intuitive to me

IMO, there's nothing intuitive about supersonic (compressible) flow! That's exactly the reason I didn't take the AE (aerospace engineering) fluids course in college, though I was really intrigued with it... I was afraid I'd have too tough a time grasping it and the advanced math that goes with it, together. But I took the two prereq. fluids courses and loved 'em!

So I admit I know very little about supersonic flow and that's why I'm asking for help!

The reason I've always been curious to test an inverted boat tail is because of what I know of subsonic flow (efficient airfoil/wing shapes, etc.).

Now, a boat tail bullet will generally have a significantly better BC than a flat based bullet of the same weight. (This is evident in all data I've seen - Sierra, Hornady, etc.) So it seems that maybe the back end is also very important supersonically. (look at all the turbulence behind the bullet in your photo.)

I can say with certainty that a typical boat tail bullet would have a higher BC backwards while subsonic, but I just don't know if it would be true while supersonic!?