Positive compensation at 100 yards.

[Tod Hendricks]

I shot through a Shotmarker at 100 yards on the way to 1000 yards in an attempt to detect positive compensation at 100 yards. Something I saw in a YouTube video.

I ran this setup many times in different test formats, from ladders to multi shot groups. There was no backer or target in the 100 frame. In my tests there are instances that appear to be PC, unfortunately for me it was almost impossible to duplicate between range sessions. I’m thinking the PC node is too narrow to reliably stay in it. If you could test this just prior to a long range relay you may have a chance to utilize it.

What I did learn is, groups at 100 yards are almost useless as an indicator of bullet grouping at 1000 yards. They are very interesting though, the grid lines in the target pics are 1 MOA. All but 1 of the attached pics are single shot ladder tests.

This was a very easy setup for me, I ran it many many times as a secondary test. It got to the point where I hardly looked at the 100 data so I just stopped putting on the 100 yard Shotmarker.

 

[SGK]

Brian Litz did an exhaustive test regarding positive compensation and dismissed its existence. (Which makes sense.) It looks to me from your targets that velocity variance (coupled with point of impact variance at 100) explains a LOT.

 

[JEFFPPC]

A constant source of amusement.

 

[tom]

What I did learn is, groups at 100 yards are almost useless as an indicator of bullet grouping at 1000 yards.

Imagine that, lol

Tom

 

[gunsandgunsmithing]

Not enough info Tod. How do you define a narrow window? And yes, if you're tuned for PC, it does change with yardage. I don't put a lot of weight on pc but I do believe it's real. I look for small, round but repeatable groups. Ideally, I have pc at the same point but it's not my goal...particularly with cf, where we have control of load quality.

 

[Captrayj]

If Tod has something to say .. it’s probably wise to listen….

 

[Ccrider]

One observation I would make regarding the 1000 yd/100 yd group data presented above is that the groups at 100 yd are not even close to what I would consider "good" groups for a well-tuned F-Open rifle, ranging from about 0.8 to over 1.0 MOA. The possible exception to that statement would be the 100 yd target from the final pair (B41), but even that 100 yd group is still larger than 0.5 MOA. Thus, I think trying to infer anything from the shape of the 100 yd groups that could be used for comparison to the 1000 yd group shape would be difficult.

This is what I was wondering about. It would be nice to see the results using a rifle tuned to shot under .2 at 100.

Still an insightful post with a neat example of using the Shotmarker and paper targets to see what’s gong on at 100 and 1000.

 

[SGK]

"I shot through a Shotmarker at 100 yards on the way to 1000 yards"

"In other words, the 100 yd parabola will not perfectly overlay the first 100 yd (or any other portion) of the 1000 yd trajectory."

He is shooting through one target onto another. One trajectory for each bullet.

The Shotmarker velocity estimation has a decent amount of error (SD) if that's where you are getting your V from. And yet when one compares relative POI at 100 and relative velocity, there's a high degree of correlation with what one would expect relative further down range from a vertical perspective.

 

[SGK]

It's actually more informative to see some difference in shot POI at 100. Certainly, however, if they all went through the same hole at 100 we'd see a tighter group (ceteris paribus) at 1000. And if they all had the same POI at 100 and the same velocity...?

 

[Jefferson]

to answer your question ned

500-600 yard tuning may have some benefit to 1000 yard groups

Notice I said may

best to tune at 1000 for group shooting at 1000

mark your bullets and fire them at the same time and compare seating depths and

AWAY WE GO

 

[Jefferson]

thanks Ned

might I suggest something that will seem silly

take 15 rounds to the line in 5 different colors on the bullets

get a new practice or slighter target put up on the board

when the go signal is given fire all fifteen in a deliberate moderate speed holding centre all the time

get the spotter to pull the target down and circle the
3 red
3 blue
3 black
3 green
3 pink shots and mark then by indicating which colors they are

send the cell phone pick to you with the results

shoot your first relay with 1 of your 2 pet loads

then get the slighter target back between relays and load to the Color that shows the least vertical

that is how to tune at 1000, maybe the match director may frown but you will then have your load for that day or weekend

side benefit is you will have an idea where Center is on the 15 rounds and use that to correct your scope to get close to Center for the next target and then go from there as per usual

I know goofy as hell but essentially you are tuning for 3 seating depths and 2 powder charges which you might do in the dead calm on practice day

if the match director and target puller are kind to you that would go a long ways to finding out what works

if it is not a state or regional championship match they might approve it

Or for a fee let folks take a practise TUNING TARGET on empty benches or part of the first match separate and apart from the first relay

I know the regular shooters will complain and say to you

just retire and come 2 days early to your hotel room like everyone else that is retired and has money

man that should spark a debate or get me a step closer to the lonely bin

and for the record

the looney bin is ok as long as your paying

later eh

jefferson

 

[SGK]

To believe it better to do load development at 1000 yards versus, say,100 (if you must, with a POI of impact commensurate with a 1000 yard target and so shooting a "1000 yard trajectory") is to suggest that (1) atmospheric variables over 1000 yards are no more a possible influence than they are over 100 yards and/or (2) positive convergence might exist. Clearly (1) is nonsense - the risk of something downrange affecting the final POI is, of course, greater the longer the flight time and distance. Believing in (2) is voodoo IMHO as it flies in the face of known physics and empirical test after test. Run your eye over the tests above. Look at 100 yard POI relative to vertical group center and observe also the shot's relative velocity. Make a prediction as to POI at 1000 - faster shots hit higher at 1000 unless they were already low at 100; shots that hit low at 100, hit low at 1000 unless they had higher relative velocity to compensate etc etc.

 

[jelenko]

the 100 yd parabola will not perfectly overlay the first 100 yd (or any other portion) of the 1000 yd trajectory

Ned - Would you expand on this?

Clearly, I can take a shot that lands at 1000 yards and it passed through the 100 yard mark. Are you saying that the path from 0 yards to 1000 yards is not a parabola?

 

[Thauglor]

The parabolic trajectory of a bullet sighted in at 100 yd is not exactly the same as the first 100 yd of the parabolic trajectory of the exact same bullet/load sighted in at 1000 yd. In other words, by changing the launch angle (and I'm using the term "launch angle" here only in its most generalized and basic form to describe the angle of the rifle/barrel with respect to horizontal; in the figure below the term "loft" is used), as we would do when changing the scope zero from 100 yd to 1000 yd, the parabolic trajectories would no longer be exactly the same (i.e. overlapping), even within the first 100 yd. The parabolic trajectory of the bullet fired with the higher launch angle (i.e. somewhere between about 25 to 35 MOA greater elevation in order to zero at 1000 yd) would be wider and taller than the one that had been zeroed at 100 yd. As an example, look at the trajectories for 10 degrees and 20 degrees in the following image (note: the y-axis dimension in this image is referred to as "loft") - again the key here is that the two parabolas are not superimposable because the lauch angle or "loft" is different.

View attachment 1496974

The reason I mentioned this is because it seems to me that there could easily be a difference in the average size of 100 yd groups fired using a load that had been optimized at 100 yd versus groups that were measured at 100 yd, but fired using a load that had been optimized for 1000 yd and a rifle sighted in at 1000 yd. I'm trying to wrap my brain around how different in size and shape I should expect the two groups to be as measured using Tod's approach described above. The two parabolas may be only minutely different from one another, but even minute differences can be multiplied in terms of changing group size/shape at 1000 yd. In any event, I'm not sure exactly how much difference we should expect between groups fired at 100 yd using a load optimized for 100 yd, and groups measured at 100 yd, but fired using a load that was optimized for 1000 yd and sighted in at 1000 yd. So I was just thinking out loud about how the differences in the two parabolas might be part of the reason that a load tuned for one distance may totally not be optimal for a different distance. That phenomenon is not as easy to explain is it might seem on the surface.

Stated another way, if a load tuned at 100 yd is unlikely to be optimal at 1000 yd, then it's safe to say that a load tuned at 1000 yd might easily not be tuned optimally for 100 yd. What should we really expect to see when shooting a load that was tuned to shoot bugholes at 100 yd versus groups measured at 100 yd with a load optimized for 1000 yd and fired with a 1000 yd zero? How should we expect those loads and groups to compare at 1000 yd? Is it possible to tune a load that is very close to being optimal for 1000 yd at some shorter distance? If the answer to that question is "yes", then I'd expect that knowing how a shorter distance group should relate to a well-optimized 1000 yd group in terms of size and shape, and whether that relationship would change depending on exactly how the testing was carried out would be important.

The bullets know nothing about parabolas or geometry. You can easily shoot a 100 yard target with your 1000 yard sight setting, just get a taller target. This way your "launch" angle will be the same. The difference could be that on the way to the 1000 yard target, your bullet has to fly higher and be exposed to different winds as well as longer exposure to these winds

 

[SGK]

As above

And all the more reason to do load development over a chrono. Small variances in velocity can be hard to see on paper at 100 but they’ll show up at 1000. Tune to both small on target at 100 and with small V SD and with good atmospheric conditions you’ll have small groups at 1000. A bad group at 100 isn’t going to get smaller at 1000 except by pure chance of a small sample size.

 

[Fairchaser]

As I understand it, the traditional method of load development is to use size and shape of groups at 100 yds and verify the load at the competition distance.

But, a more effective method might be to use PC to select the optimal load based on a speed test and then tune that load at longer distances.

If what you are saying is accurate, the groups at 100 yds have little to do with groups at longer ranges so a better method for load selection must be based on some other criteria.

 

[B Nettesheim]

Notice that his 1000 yard group is ROUND!!! , and he is doing his share of winning

 

[jelenko]

The bullets know nothing about parabolas or geometry. You can easily shoot a 100 yard target with your 1000 yard sight setting, just get a taller target. This way your "launch" angle will be the same. The difference could be that on the way to the 1000 yard target, your bullet has to fly higher and be exposed to different winds as well as longer exposure to these winds

This is what I'm thinking.

From what I've read elsewhere, I get that a load tuned for 100 yards may not be tuned for longer distances and vice versus, but it wouldn't seem to matter which 'path' it took to get to the target [other than environments].

 

[CharlieNC]

Anyone who has shot many ladder tests has observed higher charge weight/faster velocity hit lower on the target. From ballistics alone it is clear that this is not normal as there is a predictable effect of velocity on point of impact (the ballistic slope, BS). This results from the barrel harmonic vibration in a sinusoidal manner (harmonic slope,HS). The resulting point of impact is POI = (BS-HS)* Velocity which combines both factors.

Longer distance increases the BS but the HS wave is constant, meaning the best charge/velocity must be adjusted in an effort to move to the HS which perfectly offsets the BS and known as positive compensation. Because this behavior is sinusoidal, being on the opposite side of the slope results in a major deterioration sometimes called a scatter node.

While there is generally a charge/velocity node, there is no guarantee that it is wide or flat enough to offset the velocity variabiliy. But the general observation of faster hitting lower at some charge weights leave little doubt that positive compensation exists to some degree.

 

[CA_Shooter]

I shot through a Shotmarker at 100 yards on the way to 1000 yards in an attempt to detect positive compensation at 100 yards. Something I saw in a YouTube video.

I ran this setup many times in different test formats, from ladders to multi shot groups. There was no backer or target in the 100 frame. In my tests there are instances that appear to be PC, unfortunately for me it was almost impossible to duplicate between range sessions. I’m thinking the PC node is too narrow to reliably stay in it. If you could test this just prior to a long range relay you may have a chance to utilize it.

What I did learn is, groups at 100 yards are almost useless as an indicator of bullet grouping at 1000 yards. They are very interesting though, the grid lines in the target pics are 1 MOA. All but 1 of the attached pics are single shot ladder tests.

This was a very easy setup for me, I ran it many many times as a secondary test. It got to the point where I hardly looked at the 100 data so I just stopped putting on the 100 yard Shotmarker.

Thank you for sharing your test @Tod Hendricks . I think this is pretty fascinating and considering you have not seen anything consistent is pretty telling that PC may not be something reliable at 100 to 1000. It's possible other distances may be more affirmative (IE: testing at 200 or 300) but probably will amount to the same result