A few days ago, I had cause to do some digging on this topic. It seems like something worthwhile reporting here. Enjoy:

My main source is Harold Vaughn's Rifle Accuracy Facts. Harold was in charge of handling the fluid dynamics computations for our country's ICBMs, and he held a doctorate in the field. He was also a hard-core experimentalist, dragging a big old Tektronix 555 oscilloscope (I had one of those once.) and gas generator into the field to make measurements. If you want to understand how his work is regarded, try buying a used copy somewhere.

Question 1: Does the nose of the bullet trace out a spiral as the bullet flies? Yes. Harold developed software that accurately models the situation. The software is capable of six degrees of freedom, i.e., it handles position in three dimensions and angles in three dimensions. Here is one of the graphs he produced.



Question 2. Does this cause the bullet to corkscrew around its trajectory? Yes. It must be so, because the bullet is "coning" and the drag vector always points from tip to center of base. When the bullet nose does not point exactly along the trajectory, the drag vector has a transverse component that moves the bullet sideways and up and down. Fluid dynamics substantially changes the situation, vs. ballistics in a vacuum.

From page 182:


Question 3. Is this effect important to most of us? No. From page 192:


Bryan Litz's money (he issued a challenge for anyone to show a rifle more accurate at 200 yards than at 100) is probably safe unless someone with a lot of patience and a superbly accurate rail gun chooses to compete for it.

Awesome post! This is what I love this place!

Bookmarked.

It's "corkscrewing" around an imaginary mathematical point.

The graph is misleading since that's not the bullets path.

It's the motion of only the tip and it all occurs in fractions of a second.

Not really.

There is nothing imaginary about the centerline of the trajectory. It's real. I can tell you what it is and where to find it.

There is nothing misleading about the graph. It's clearly labeled as the pitch and yaw of the bullet.

The corkscrewing is a consequence of the pitch and yaw. It is real motion of the center of mass of the bullet around the centerline.

One of the first, and I thought one of the coolest, articles I can remember reading in one of my cousin's American Rifleman was about this very topic. There was a drawing that looked like a drawing a kid would make with a Spirograph toy almost like petals of a daisy similar to the one in the OP.

I've been hooked ever since and that was probably around '62, age 12.

And the drag induced by the pitch and yaw of the bullet passing through the atmosphere. As was pointed out, this motion would not occur to this degree in a vacuum.

Yes!

If you could somehow magically turn the nose of the bullet by say half a degree to the right, and hold it there, the drag vector would have a leftward component, and pull the bullet to the left.

Denton,....GREAT POST !

.......There's absolutely NOTHING that compares to having good rested eyes, and a quality optic,....while spotting / coaching for a well trained and capable rifleman, on a cool damp early summer morning, with the sun at ones back.

The given being that your accomplished, "Across the Course / 1000 yard = / F class shooter has loaded his ammo to either be sub sonic, or still supersonic anywhere's CLOSE to the target he's ranging in on. "Going through the wall", just prior to hitting, PARTICULARLY at ranges beyond 500 M. is bad mojo.

It's just awesome watching the trace enlarge and leave that "corkscrew" and than "go back to sleep, and drop RIGHT INTO the 10 and X ring.

The Ranges at Raton are SUPERB for this ,....particularly with the big BPCR and BPLRT guns and loads.

Love to shoot,....I mean LOVE it,....but'spect I might enjoy spotting almost as much,....maybe even a touch more.
....and I love either in a howling switchy gale of wind, too.

GTC

I'll be first: at least 4 pages but no more than 6.

...yup, and one of the reasons that a well qualified spotter will ask his shooter whether he's shooting a right, or left hand twist barrel, right off the bat.

GTC

Mr D

A very interesting read and enjoyable. Thanx for kicking it off.

Jerry

I'd love to see high speed video of this phenomenon.

Thanks Denton,

But how dare you bring science to the 'fire.

Next you'll be attempting to prove that the bullet doesn't rise when it leaves the muzzle.

Geno

I'm reminded of the story about someone who claimed that a curve ball is an optical illusion. Some pitcher said that he should go stand behind a tree and he'd kill him with the illusion of curve balls.

So how does this pencil out in inches of deflection on the x and y axes?

I do find it a little odd that the pattern is essentially round. I would think gravity might alter things into more of a vertically shaped ellipse.

I like the sarcasm.......we all know the bullet has a mid trajectory arch

Jerry

About plus and minus .009" for the example shown, close in, diminishing to plus and minus .003" farther out.

If this is your biggest accuracy problem, I'm impressed.

Thanks. Now that I have wind and gravity mastered. I'll start taking that into account the next time my Sharps goes to the 1K line.

Dang, now I know why I missed that big buck last year.....

It's still really meaningless since it's only a few thousandths of an inch and takes place over a fraction of a second in most cases.

The context of the discussions where it always comes up is some try to claim it explains the fantasy that some rifles that aren't accurate at short range are accurate at longer ranges.

Otherwise it's just trivia for most small arms ballistics.
It may play a slightly larger role in artillery fire when distances are measured in miles rather than yards.

Interesting hypothesis. But that's all it is until someone can take Brian's money.

Some like to ponder and learn about these things while others seem to be prone to sticking their fingers in their ears while hollering so no one else can have the discussion.

I am for having the discussion and will gladly take whatever I can learn, whether it seems to have practical application or not. The world just really doesn't work the way we believe we know it works.

I'm sure I've missed at least one antelope due to the gravitational effects of Dark Matter.

No one is stopping anyone from discussing anything they want.
"Discussion" doesn't mean "nod in agreement" though.

The OP is copied from a long discussion on another site, and the context there was the claim that a rifle can be more accurate at long range than at 100 yards.

Some seemed to think this data, and some other data on barrel vibrations amounted to some sort of proof it was true.

None have actually demonstrated that on the range.

If you want to "ponder and learn" go over to Applied Ballistics and read from the source.

No one is protesting disagreement. It's every bit as much a part of discussion as agreement is. But, the subject being discussed is not whether the phenomenon is worth talking about or not, and the thread should not be sent off in that direction.

And I believe I'll ponder and learn where ever I please, but I do thank you for the additional resource.

Bravo !

We've had some good laughs over the years at
"Snippers ballistics 101" lectures and assertions.
......some of that series goes off into pretty strange territory.

wouldn't pay a whole lotta' attention to his horsechit.

GTC

Shame on you.

You know very well the the post is not copied from another site. You know very well that I posted the same material under my own name on both sites. In the context of my post at The High Road, I think my post pretty well put to rest the notion that the bullet corkscrewing along can account for someone getting much better groups at 200 yards than at 100. Those who are interested can see the whole discussion here.

Whether you believe Harold Vaughn's analysis or not is a matter of great indifference to me. It is based on sound physics. If you have a more compelling analysis, I'm sure we'd all love to see it.

I'll go put on the popcorn,.....

GTC

Interesting info Denton, thanks!

And this begs the question(s), what bullet, what twist, what rifling, what crown, and what combination thereof will minimize corkscrewing of the bullet?

Knowing the answers will help me minimize all those misses...


Casey

Interesting, I wonder how caliber, velocity and bullet type/weight affect the pictured graph.
Please excuse my ignorance.....but does a bullet rise above the line of the bore after leaving the barrel? I've heard arguments both ways, but never from someone who actually knew...just assumptions.

It's a common question.

The forces that act on an aircraft are gravity, drag, lift, and propulsion. Because you have lift and propulsion, an aircraft can rise. The external forces acting on the bullet after it leaves the barrel are just gravity and drag. There is no lift, and no internal source of propulsion.

Per the discussion of the bullet corkscrewing through the air, I suppose that you might construct a situation where a bullet momentarily gets a few thousandths of an inch above the bore line for an instant. But in general, the answer to your question is no.

I know. I have that problem all the time with my Allen pepperbox.

Is this why my 284 doesn't group well at 100 yards but 200 and beyond it groups great? Out to 1000.

One thing I used to hear from the bench rest crowd was the bullet has to "Go to sleep". Never really knew what that meant.

I believe it means it settles into a smoother form of flight and i will for lack of a better word, say it is not as erratic and has stabilized.

But I am no rocket scientist.

It's "copied", even if you posted the original there first, since the posts are identical.

You "copied" the data from another website too.

There's nothing derogatory about it.
It's just a statement of the facts.


I didn't say you thought it was proof.

I said "some" thought it was because they cherry-picked phrases and took things out of context.

Like I said before, it's interesting trivia, but really has no practical meaning.

No, it explains why what you claim isn't possible.
Bryan Litz wants to see you shoot to prove it's true.
He will pay your expenses if you can show him.

For 200,000 years most of the experts agreed that the sun rotated around a flat earth. The concept of a round earth rotating around the sun is a recent discovery in the grand scheme of things. Rifles have been in common use less than 200 years. I think it is quite possible that we don't really know everything about what is going on after the bullet leaves the muzzle. Or even before.

I don't claim to have any answers, but enjoy the debate. All I can say is that I've had quite a few incidents where a rifle would shoot no better than 1 MOA at 100 yards, yet 3/4 MOA was quite common at longer ranges. Group size at longer ranges certainly wasn't smaller. But getting 1.5-1.75" groups at 200 when no better than 1" at 100 yards does get noticed.

Good post Denton, always good to see superstition skewered by the facts.

Another fella who made a good effort at dumbing down the science to the level an average Joe could grasp was Robert McCoy in "Modern Exterior Ballistics".

Going to sleep theory is not all that mystical actually. I take no position regarding the issue but note the graph on the OP is more than just a spiral. There are two modes of precession displayed, one the slow cycle and the other a fast cycle, indicated by the small dips in the larger arc. Fast cycle precession stems from bullet imbalance on the axis of rotation and does null in fairly short order. The larger arc, or the slow cycle precession diminishes to some degree as a function of Sg, but it does not null.

Where this puts the bullet on the target is for you fellas to debate. Beer helps.

Does this phenomena have anything to do with what Litz wrote about Nosler's LRAB BC (and his BC bullets in general) being overstated if the bbl is "under-twisted" or the bullet is slightly under-stabilized?

.277/150 is listed @ .625 but I believe he said that's only true through a ~1:8 twist and a standard 1:10 .277 bbl yields something in the mid-high .5's for BC.

Hoping I didn't start an all new argument with the above ?'s.

There could be a number of factors in that, equipment and shooter related. But the fact remains that with that rifle alone my groups measure less by MOA at longer range than at 100. Not by a great deal though, but it is consistently there. If it is a parallax issue then it is not enough for me to see.

So, when I got my chit together it will shoot 5 shots at about .2 but then only .3 at 200. Of course that is on a flat calm day.

Rifle is a Winchester 70 with a PAC NOR Super Match 28 inch in .284 with a .315 neck. I had PAC NOR mount the barrel. Scope is a Sightron SII 6-24 dot recticle on a 20 MOA base.

The load is a Berger 168 grain VLD as best, Sierra's come close but not quite as good.

I'm not familiar with Litz's position on BC as a function of gyroscopic stabilization, but I did go to the Berger page where Bryan's stability calculator is posted. http://www.bergerbullets.com/twist-rate-calculator/

The SG that the calculator produces is the same number as the GS of 2.98 used in the example in my original post. GS must be at least 1, and factors just above 1 produce corkscrewing that does not diminish well. Bryan holds that the stability number must be above 1.5 for reliability, and Harold Vaughn gives quite a discussion on how marginally stabilized bullets might work fine at altitude, but fail at sea level.

As you point out, Bryan does use the stability factor to evaluate whether the bullet is truly living up to it's BC.

So I guess the answer to your question is that this is all connected. But I'll be darned if I can provide much useful information beyond just that.

The way I look at it a bullet in flight that is not optimally stabilized due to twist rate is exposing a larger frontal area to the atmosphere, thus reducing its effective BC.

At altitude the air is thinner/less viscous so the 'effective larger frontal area' is not as pronounced as it would be at sea level.

Knowing the MV and 100 yard velocity one can extrapolate the effecivet BC for the conditions tested. The actual as opposed to calculated mv at the target would be nice to know but the LabRadar will give mv and 100 yard velocities.

An example is the Hornaday 6mm 105gr HPBT has an advertised G7 BC of 0.253. Under my test conditions it has a G7 of 0.233. 1:8 twist, mv 2850 ft/s, at 2880' elevation.

Accurate muzzle velocity and G7's begets an accurate external ballistic chart.

That sounds right.

And it sounds like the LabRadar is giving you some very useful information.

Very interesting denton. I've come across similar information years before in a book but, I've forgotten where.

Been called a liar a few times, but no matter, don't have the upper anymore to prove it... AR15.. 7 twist Krieger. Keyholed slightly at 100 yards. Moved to 200 and 300 and the bullets no longer keyholed. This was playing with 90 JLKs YEARS ago... moved to a 6.5 twist and never saw that again.

Tended to be as accurate to a bit more accurate MOA wise at 200 and 300 than it was at 100.

Also have shot a given load at a 300 yard reduced match with 600 yard ammo and the accuracy at the 300 yard match left me unhappy. Again with 90 JLKs early on. Learned that it was just best to leave them for actual 600 and beyond matches...

Keyholing at 100yd, accurate at 200 and 300yd....? More accurate at 600 than 300?

Litz would love to talk to you...but of course, the upper isnt around anymore..

I think it's important to clarify whether we're talking about absolute accuracy, as in group size in inches, or relative accuracy, meaning measuring groups in angular terms like MOA. A 0.5" group at 100 yards is ~ 0.5 MOA. A 0.75" group at 200 yards, while larger than the 100 yard group, is only ~0.375 MOA. So it's relatively more accurate (MOA), but less accurate in regards to absolute group size (in inches).

I think the Gyroscopic Stability Factor is confusing to some.

The graph is a cross section showing the start and end point.

At say, 100 yards area of the graph is 'x' this gyroscopic instability may not get worse as the bullet continues down range.

So, if at 100 yards a 1" group is produced this doe not mean that at 300 yards a 3" group will be produced.....but it will not be less than 1" at distance.

The group size in inches will not get smaller at distance but expressed in MOA it can, so one moa/100yds may be 0.8 moa at 200 yards.

From my notes, a 155 gr target bullet in a sporter rifle will put 3 into one inch at 100 yards, at 1000 yards it will put 3 in less than 10", actually 7.625"or ~0.7xx moa.

Accuracy did not improve ...it just didn't get any worse as the gyroscopic stability did not degrade in a linear fashion.

Maybe it causes the misses I sometimes have??

Interesting stuff. Perhaps the corkscrew effect has something to do with your assault bullets? .... http://order.politifake.org/image/p...iberalism-mental-politics-1471665449.jpg

I'd love to see a quantification of the relationship of the maximum deflection and GS.

If the 0.009" deflection occurs at GS 2.98, what is the maximum deflection at a GS of 2 or 1.5 or 1.2....?

Unfortunately, I have neither the understanding nor the tools to accomplish the analysis....

Accuracy is a matter of placement relative to POA.

Precision is the term used in reference degree of dispersion. Dispersion is just that, whether it be measured inches/mm/angstroms or MOA.

In Vaughn's book, he showed the pitch and yaw for a less well stabilized bullet, and the initial amplitude was about the same as for the well stabilized bullet. The difference is that the pitch and yaw did not die out much over the trajectory. Since the corkscrew effect follows from the pitch and yaw, I suppose that the big difference would be that in the less well stabilized case the radius stays about the same for a longer time.

That's about as much calculation as I'm good for these days...

I'm aware of that, and have been a bit bothered by the industry standard terminology myself, at times, but that's simply the way it's spoken of across gun culture. Manufacturers don't make a precision guarantee, they make an accuracy guarantee (guarantee that your scope will be accurately zero'd?? ). I was more referring to the fact that Bryan Litz and others are talking about a rifle shooting better at longer distances than shorter ones. Are they talking about absolute accuracy or relative accuracy? I've never seen a rifle that shot smaller linear groups at distance, but I have seen and owned rifles that shot smaller angular groups as the distance increased.

Being old school I'm not surprised that this would be new knowledge to those purporting to be rifle shooters!

Could the scarring on the bullet from the lands and grooves affect bullet flight?

Bryan's contention is that a rifle that shoots a 3/4 MOA group (for example) at 100 yards will not have that same group measure smaller than 3/4 MOA at 200 yards and beyond. IIRC he tested 74~78 different load/bullet/caliber combinations and found one test (.223) that was questionable.

Of COURSE it does,....and rifling direction ( Right or left) rules that phenomena, before wind even entersin, ....which it does.

Look at a Buffington Sight on a Trapdoor Springfield, and go figger.

Rick's right,....there's been a LOT of knowledge lost along the way.

GTC

Thanks.

That corkscrew flight is a design feature. It prolongs the bullets surface to flesh contact patch and thus imparts more dynamic force. It also transfers rotational force to the viscous internal structures of the game animal, resulting in greater hydrostatic shock.

Magnus effect. Side wind. Please discuss.

As far as I understand the Magnus Effect, it applies to a spinning object whose axis of rotation is across its trajectory. Since the axis of a spinning bullet is along the trajectory, I don't think that Magnus Effect from forward motion of the bullet is important.

You might argue that wind blowing across the trajectory satisfies the conditions for the Magnus Effect, and it does, but the effect is too small and in the wrong direction to account for vertical wind drift.

Ordinary wind drift is mostly horizontal, but also has a smaller vertical component. The vertical component is from a vertical component of the drag vector, not the Magnus Effect, though you will get hot debate on that from some quarters.

That takes it to the limit of my understanding. Or maybe beyond.

Old farts that have been shooting rifles two days before the dirt got here know this stuff!

They're called RIFLE CRANKS!!

Sounds like all the fuss is about a factor that while true in theory, is so small it gets lost in much larger errors caused by other factors. Like most of the things that provoke internet arguments.

Good luck eliminating every other source of error so the 6 thousandths of an inch difference becomes apparent.

Yeah, that was my thought.

I also just reread this thread and strongly suspect some who've posted are confusing spin-drift with the "corkscrew" effect.