I'm going to attempt to arrange a 100 yard target such that the bottom of is will cut across the center of the 400 yard aiming point. Lay of the land prevents this using 100 and 300 yard targets.
The idea will be to hit both the 100 and 400 yard targets with each shot. Then use the 100 yard aiming point then the 400 yard aiming point.
We'll see if I can bring this off...
Roy,
It will be interesting if you can set up a shoot thru target of your own. Here's a couple tips. First, you'll definitely want someone to help you place the targets, and walkie talkies to communicate. Second, you'll want a large target for downrange, probably larger than you think. My 100 yard paper is about 10" wide and I shoot groups from the left to right edge, which means at a minimum, there's 30" of width that needs captured at 300 yards, and that's if everything is perfectly centered.
Hopefully you can get it set up and shoot some successful tests. The more eyes and brains on this the better.
DocB,
Can you elaborate on your experience at Ft Hood? What kinds of rifles, ammo, ranges, and positions were you shooting? So far I've tried everything I can think of including what people have recommended based on their observations, and I've not been able to find a combination of rifle/bullet which produces converging groups.
To answer your question about the shoot thru target, it's better explained on the first page of this thread. Here's a picture:
Note this was one of my earlier attempts, before I figured out the downrange target needed to use a much larger paper.
Matt,
You bring up several points which I'll address separately.
Your account of benchrest and 'heavy guns' occured to me this past summer. I downloaded the results of the most recent supershoot, which contains the overall averages for over a hundred shooters over several days at 100 and 200 yards. Loading the results into excel allowed me to do some statistics on the 100 yard aggs vs. the 200 yard aggs. I thought this large amount of data might contain a clue about large scale trends vs. isolated incidents of group convergence. What I found was that the average MOA group size at 200 yards was larger than at 100. There were some individual cases where someone agg'd better at 200 than 100 (in MOA), but those cases were rare and easily explainable by statistics. The overall conclusion was that the BR rifles did not exhibit converging groups based on the 100 and 200 yard match results.
One flaw with this investigation is that the results of the BR match were obtained from shooters firing loads that were 'tuned' for 100 yards at 100 yards, and loads that were 'tuned' for 200 at 200. Had they all shot their '200 yard loads' at 100, then the results may have looked different.
I have a rail gun which I plan to use on the shoot thru target to test this first hand. Problem is, currently my bench in the lab is wooden, which isn't an effective platform for the return-to-battery system. One of my winter projects is to replace the wood bench with a concrete one with pillars that go into the ground and provides a stable platform for the rail gun. The concrete bench will be good to have in place, but I don't consider it necessary for studying converging groups. If groups can indeed converge in their flight path, it will be evident on the paper 'shoot-thru' target system regardless of the platform. If the groups only appear to converge based on aiming at 100 vs. 300, then this disproves converging groups and points to an optical cause.
Regarding the ballistician at Sierra, it seems strange to me that a particular bullet could be identified as the 'fastest settling' bullet considering that barrel twist affects the stability of all bullets. If this individual told you specifically that he's observed converging groups, then I would like to get in touch with him to discuss his observations.
As for the match shooter who knew the old guy who could see bullets corkscrewing; again, if you could put me in touch with that guy I'll be happy to talk to him as well.
Matt, You stated:
Yes a lot of times the MOA is smaller at 400 then 100. Most of my match guns will shoot in the 1/2 to 3/4 inch range and at 400 where I do most of my testing will shoot 1 to 1 1/4 inches. It does this on a fairly consistent basis.
Perfect! This is what I was hoping to happen on this thread, to find someone with a rifle that consistently exhibits converging groups. Sir, would you be willing to travel to Michigan and demonstrate this in my ballistics lab? (Please see the original post which started this thread, I'll pay your way). I've tried dozens of combinations of rifles, barrels and bullets (many of which have been recommended by people who say they've seen converging groups with that equipment) and so far none of them have actually produced converging groups. If someone could demonstrate converging groups
on a shoot thru target, then and only then can the cause be directly investigated.
Darn, this shoots down my incentive to fab up a machine rear for this project.
Still may do it though.
Considering Matt's post what is going to be the fastest means to the end?
Roy, I would still encourage you to perform your own experiment. Hopefully Matt will make it to MI and demonstrate his equipment, but if not, it's good to have multiple sources experimenting.
Alex,
Thank you for sharing the Kolbe article on positive compensation. I'm aware of the concept and this is probably the best article I've seen on the subject, mostly because it includes live fire experiments.
For those who aren't familiar with the concept, 'positive compensation' is when a rifle/load produces a condition in which the 'faster than average' bullets are exiting the muzzle when it's at the bottom of its oscillary motion (or when it's traveling down), and the 'slower than average' bullets are exiting the muzzle when it's at the top if it's cycle (or moving up). In other words, the system is self correcting in the sense that faster bullets are essentially pointed lower. At short range the group might be tall due to the oscillation of the barrel 'throwing' shots high and low. But at some range, the velocity effects make up for the muzzle pointing up or down and the group 'converges'.
This is a documented and accepted mechanism for converging groups, but it doesn't provide complete closure on the issue.
For one thing, this mechanism could only produce converging 'vertical' groups. Since gravity is one of the key forces which makes positive compensation possible, there's no equivalent force in the horizontal plane to make groups converge in all directions (which is what many shooters claim to have observed).
Another reason this mechanism doesn't fully explain the observations of many shooters is that many accounts of converging groups are between 100 and 200 yards. Considering the velocities that centerfire rifles typically operate at (the article was based on live rimfire testing) the amount of barrel whip and MV variation that would be necessary to make a group converge from 1 MOA at 100 to 1/2 MOA at 200 would be extreme.
In summary, although barrel harmonics and MV effects can cause group convergence in the vertical plane, it doesn't fully satisfy the inquiry.
Most of the time I hear of this phenomenon from shooters flinging the heavies...300gr .338s or the various 375 and 408 cheytac rounds. It comes up enough that I know that good shooters are observing the effect, I just wonder about why it happens. It would be cool if someone with a rifle that does this a lot would run this test and help figure it out. Might lead to a great understanding of external ballistics, better optics construction, shooter technique or a combo of those.
I agree, which is why a lot of my testing has been with 338 and 375 caliber, and even with a platform which is commonly observed to produce converging groups (the Desert Tech HTI). So far it's failed to produce a single converging group on the shoot thru target.
Did you read the article?
The reason for larger moa groups (vertically) at shorter range is explained in the article and Varmint Al's web site.
P.S. barrel harmonics and bullets going to sleep are not two different things, ever notice how you can tune the size of your bullet holes with the load?
Alex,
What Mr Neary is discussing in the video is simply marginal bullet stability. The very slow twist barrels used by BR shooters produce bullet flight which is at the very edge of stability to the point where slight changes in MV can be the difference between round holes and oblonged holes (due to the bullet flying straight, or with yaw). Barrel harmonics is a real thing, which is determined by the barrel length, contour, material properties, pressure spike, etc and which produces known effects. However, bullet stability is another, unrelated thing which is determined by twist rate, mass and aerodynamic properties of the bullet, velocity and atmospherics. Barrel harmonics can affect how much initial yaw the bullet leaves the muzzle with, but then stability will determine how quickly the bullet is able to dampen that yaw.
The discussion of barrel harmonics and bullet stability is usually linked to the topic of converging groups by the idea of epicyclic swerve. In other words, the bullet is described as having a 'corkscrew' flight path which is large at close range, then settles at longer range. Although sound in concept, there is a major problem with the 'corkscrew' theory. To the point; yes epicyclic swerve is real, and does steer the bullet in a 'corkscrew' path, HOWEVER, the most divergence that the bullet experiences is on the order of 0.030". This is based on extensive 6-DOF modeling in which bullet stability, initial yaw rates, velocity, and every other parameters are swept as inputs to the simulation. Basically, the reason why the bullet doesn't steer very much is because its coning motion is such a high frequency that as soon as the bullet begins to steer right (due to the nose pointing right) before you know it the nose is pointed back left and very little net deflection can occur. Here's an animation I made of a bullet showing its angular pitching/yawing as well as it's flight path:
This video is often misunderstood. Note that the illustration on the left is not showing the path of the bullet, it's showing the angle that the nose is pointed. the view on the right is showing the bullet path from the shooters point of view. You can see how tiny the actual 'corkscrew' path of the bullet is.
This video is the result of modeling which means it's only as accurate as the inputs, and there are a lot of inputs needed to make a 6-DOF model work. I'm sure that there are some imperfections in some of the inputs. However, when you consider that the model would have to be wrong by more than 100 times in order to show the level of group convergence that's commonly claimed (1 MOA at 100 and 1/2 MOA at 200), it becomes far fetched to accept epicyclic swerve as the explanation.
Another problem with the epicyclic swerve (corkscrew) theory is this. In order for epiciclic swerve to cause larger angular groups at short range, and smaller groups at long range, it would be necessary for the bullets to
always pass thru the 100 yard target on the outer edge of their 'orbit', and
always pass thru the longer range targets on the inner edge of their orbit. This combination is statistically unlikely to repeat often enough for a shooter to observe it happening "all the time". Even if you think of the corkscrew existing at 100 yards, but diminishing at 200, you would still need the bullets to pass thru the 100 yard target on the outer edge of their orbits to produce a larger group.
If someone knows of another mechanism by which barrel harmonics and bullet stability might explain group convergence, I'm all ears. Epicyclic swerve is the only thing I know of, and that theory doesn't hold water.
Canadian Bushman,
Awesome! Thank you for doing this test and sharing the results. You're seeing the same thing I've been seeing, which is a lack of group convergence. Can you tell us; is this a rifle that you had suspected of converging groups based on shooting it at 100 yards vs. longer range?
As for the bullet hitting the paper, I think what you might be hearing is the sonic crack (of the bullet, muzzle blast or both) reflecting off the paper. I was also concerned about the effect the paper would have on the bullet with the 'shoot thru' target. However, back in the day before technology (optical bullet sensors and 'spark' ranges) ballisticians used 'yaw cards' to study the pitching and yawing of bullets. Yaw cards are little paper cards which are carefully placed near the muzzle. They would set them up so the bullet passed thru dozens of these cards and they would measure the angle of the oblonged bullet holes for example. They didn't believe that the bullets hitting the cards effected their flight in a signifficant manner, and later high tech optical sensor based testing confirmed the results of the yaw card firings. Another thing that puts my mind at ease about the paper is that the groups at 300 yards are always an enlarged version of the pattern seen at 100, which indicates there is little to no deflection. Finally, if there 'were' an effect, it would be to cause more dispersion on the 300 yard target. So if the shoot thru target ever produces evidence of group convergence, then we'll *know* it's real.
It's not an easy test to set up, so again, thank you for the effort.
All,
It's good that this thread has stirred a constructive discussion on the topic. Sometimes this subject can devolve into argument with some people saying they see the effect, while others claiming it's not possible, etc. My intent here is to get to the truth of the matter. Anecdotal accounts are useful in the sense they can give us ideas of where to look, but the standards of science are more demanding. In order to learn the root cause, the effect has to be isolated and demonstrated repeatably. Then, once we see the thing happening, we can begin to formulate theories. The problem with so many internet discussions is that many theories are formulated and argued about before it's been established for sure if the observation is related to bullet flight, optics, or just small sample sizes of 100 yard vs. longer range shooting.
Point being, although it's fun to discuss what might cause groups to converge, I'm still at step 1; which is to determine first if they really do.
To this end, the 'shoot-thru target challenge' still stands. If anyone is certain they have a rifle which can demonstrate this effect, I'll pay your way to the Applied Ballistics Lab in Michigan.
Of course this problem deserves a solution, and sheer curriosity is justification enough. However, there is very important utility in fully understanding this topic. The current acceptance of the 'converging groups' theory means that if someone has a rifle which shoots poorly at 100 yards, many experts will tell him that's just how it is, but no-one knows why. If we can get to the bottom of this and discover, for example, that it's not a mysterious bullet flight issue but rather an optical issue, then we'll *know*
exactly what to do when we see this happening. In other words, it will be a known problem with a known solution, similar to the understanding of vertical stringing at LR being caused by MV variation. If we discover that it is a bullet flight issue, and get to the bottom of what it is, then we'll learn what to do to avoid the problem.
One thing is for sure; the 100's of internet discussions have failed to produce an explanation yet. Let's try science.
Sincerely,
-Bryan
