Update to "Beyond 100 yards"

[KN]

You all remember the debate, if not it can be seen at http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=89507

I thought I would start another thread instead of lengthening the old one.

I got two more e-mails from Sierra bullets about group size being better at long range vrs short. One from Ted Almgren, and another from Carroll Pilant.

Ted's reply first,
"Hi Kevin!

I e-mailed Carroll a potential explanation for the smaller group sizes at longer ranges.  I suspect that it will create a number of questions, some amazement and some doubt when you receive it.  The mathematics and our experience confirms that it works that way however.  Yes, the groups could be smaller at longer range and yes, some bullets cone at shorter ranges and the coning damps out at longer ranges due to primarily aerodynamic effects.  You should note that many, many variables affect the bullet flight, a large number of them unpredictable.  One thing I noted when I re-read the thread this evening was that some participants may assume that the bullet flies in a vertical plane to the target unless affected by wind.  It does not.  The aerodynamic effects and an effect called spin drift cause the bullet to drift horizontally, with increasing effect, even in the absence of wind.  These effects are taken out (compensated for) when you sight in at a specific range.  If you sight in at 100 yards and then shoot at 1000+ you can see some of the horizontal effects.  If you sight in at 300 or longer, you have already taken a portion of these effects out (at that range).  The really long range shooters (1000 – 2000 meters) see these effects and several others (Coriolis for example).

 

I will try to take the time to answer if you have specific questions but we have a number of projects for Sierra and Leupold that need to be finished by late summer so I cannot guarantee prompt answers.  The easiest way to get quick and much better practical answers is to contact Carroll.  I learn something new from him every time I talk to him.  If he doesn’t feel comfortable with the mathematics, he will forward it to Dr McDonald or I for an answer.  

Thanks for your question, I needed the review and I always learn something new when I prepare an answer!

Ted Almgren"
 
Now Carroll's reply
"Kevin, this is what I was supposed to send you. It is a little more in depth than the other.

Carroll

The "phenomenon" of smaller target groups at longer ranges than those at shorter ranges.
There are observed cases of larger target groups shot with certain bullet/cartridge loads at shorter ranges than those shot at long range with the same bullet/load.  Actually there are physical reasons why this can occur.
First, some bullets, typically long with respect to the diameter,  "cone" for a number of yards after firing with the coning damping out after 50 yards or more.  We have observed this phenomenon during our Ballistic Coefficients firing tests by noting lower and more scattered Ballistic Coefficient values measured closer to the muzzle than those measured at longer distances.  The muzzle velocities were adjusted slightly so that the velocity through the measurement screens placed the measurements very close to the same point on the drag curve.  Actually the "coning" phenomenon and it's damping at longer ranges is are discussed in some detail in chapters 9 and 10 of Robert McCoy's book "Modern Exterior Ballistics" as "epicyclic" pitch and yaw motions as they apply to a number of projectiles including the Sierra .308 168 gr  International bullet.  Indications from McCoy's testing and studies using a 6 degree of freedom analysis, indicate that during this period of large "coning", the bullet actually flies a helical path about the flight path vector.  The "Flight Path Vector" begins to deviate from the Launch Vector (boreline) due to the aerodynamic effects immediately after the bullet leaves the bore.  These effects are primarily caused by aerodynamic forces and gravity acting on the bullet.  The bullet following this helical path and coning, explains the larger target group at closer ranges.  Theoretically the bullet could be hitting the target at different points on the helix as a result of the variability in muzzle velocities of each shot.  The helical flight path postulation for both long and short period epicyclical motion is shown in figure 10.4 in McCoy.  There is another effect that causes the bullet to "drift" right for right hand twist guns called the "yaw of repose".   This effect (spin drift) is small at shorter ranges, growing to as much as 11 inches for the 168 gr at 1000 yards from a 10 twist barrel. (McCoy page 198)  This effect is typically taken out during the sighting process at a given range.  The "coning" and helical flight path effects are interactive with this drift.
In summary:  Some Bullets may print a larger target group at short Range than at long range.  This phenomenon is based on the physics of the aerodynamic, and gravity forces acting on the gyroscopically-stabilized bullet.  The actual flight path of the bullet is not just a simple smooth curve, it is actually a complex curve that consists of long period epicyclical motion, modulated by short period epicyclical motion (both of which tend to damp at longer ranges) all about a flight path vector that changes as a result of "spin drift", gravity and winds. "


Now you all are free to believe whatever you like but so far Sierra has done nothing but give me good info over the years whenever I ask.   KN

[nomosendero]

Carroll explained this situation well as far as I am concerned. I read the previous thread on this subjuect just now. Thanks & it was interesting reading the opinions!

[Graybeard]

This is me waving the great big BS FLAG.  I understand the theory of what he is talking about and to some tiny tiny fraction it might even be true. But to say the effect is such that it is greater than the diameter of the bullet hole in the paper is sheer BS.

You guys believe in fairies if you wish, matters not to me. But under no circumstances will any group EVER get smaller the further from the end of the barrel it gets.

Put it to the test if you wish. Shoot thru multiple Oehler accoustic targets which are capable of measuring group size more closely than we can do after the fact on paper targets. Do it 1000 times and not one single time will any group ever be smaller at longer range than it was any any point closer to the barrel UNLESS you luck up and a stray breeze happens to move a bullet back toward centerline.

To state otherwise is pure BS of the highest degree.

[nomosendero]

I guess that is why I did not state otherwise. But the yaw he mentioned
explains to me the targets that I have shot where the group size did not
"grow" as I expected it to at longer ranges. I bet if I look at those targets more closely I may see a very slight oval shape to indicate a slight Yaw.
No doubt, there is no magical force bringing those bullets back to the center of the group as range increases.
No, my targets aren't smaller at longer ranges unless I shot a smaller group that particular time (it happens), but allmost the same size on out there sometimes & I have often wondered about it & no it was not parrallax.
GB, I never said I bought the whole story, but I do see a partial explaination for the matter I just mentioned. But then I see some groups
from say 300 to 400 yards with other guns just fall apart.

[KN]

Look at the videos at dogbegone.com. There are a couple that are slowed down to a crawl and you can accually see a vaper trail behind the bullet as it flies toward its target. The trail will show exactilly what is happening. It leaves a spiral trail as it goes down range. It can be seen best in the still images. Especially the one labled," Can you tell who's day is about to be ruined "  

Back to the disclaimer, I never said this is the norm for groups to be better at long ranges, Just that it does happen.   KN

[nomosendero]

KN, remember that there are other factors such as wind, heat & turbulance behind the bullet that influence the trail that you see.

Again, the basic thought that Graybeard mentions has to be true, that being that there cannot be a force out there that makes these bullets return to center, especially the big group size differences that I have heard mentioned, but I have never seen.

But, as I stated in my previous post, the yaw part makes sense to me &
could explain these small differences that I have seen, namely some of my groups that have been VERY CLOSE to AS SMALL at 300 yards as 100 yards, for example. We have seen a spinning top that starts a little fast
that yaws a little & then gets stable. If that bullet starts off the same way,
then the point could yaw around that center point & then settle. well, I would expect groups to double for example at 200 yards instead of 100 yards, but with some rifles & some loads they don't.

I personally have not had these groups that are alot smaller way out there like some people who insist they have, but I have had the situation many times where the groups would be quite consistant as yardage increases up to a point. I think Carroll touched on it & I am really going to study those bullet holes when this happens, who knows. But some of the other parts of his explanation, well I am not ready to buy as yet.

[Coyote Hunter]

Its perfectly logical that bullets may wobble a bit around their central axis.  I can think of two things right off the bat that would cause this – not being straight with the bore when fired and having a center of mass that is not exactly on center.

Neither of these will cause 1” group at 100 yards to shrink at longer ranges.

I can, however, think of a couple things that will cause group sizes to shrink at longer ranges:
1. Better fit between target type and sighting device.
2. Random chance.

Both have worked for me.

[KN]

Well I can see that this will fall into the "I'll believe what I believe" catagory since there is really no way to practically prove it exsists in the real world. So I'll just say to each his own and I'll leave it alone.   KN

[Graybeard]

BUT THERE IS, that's what I keep telling you.

Put Oehler Acustic Targets at 100, 200 and 300 yards and hook them up to a PBL-43. Shoot thru them all at a target at which ever range makes you happy. Just be sure they are lined up such that the bullets will pass thru each.

In this way you can know precisely to at least 0.01" the group size of each string at each range with the exact same bullets being in the group at each.

When I see someone set this up and show it happens regularly other than as I've said I'll listen. Until them logic and physics trumps BS.

[KN]

Thats my whole point! I never said it happens regularly. It's very rare. KN