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Any thoughts on the causes and fixes for these shot patterns?

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RScott

RScott

Member
Dec 28, 2020
145
102
FL, United States
These are 25m benchrest shots. Very calm conditions. 15mm between POA bulls. They were shot left to right (row 14 first), using .177 JSB 13.4gr MRDs (washed, not lubed), using my Daystate Pulsar (with Heliboard and Falx moderator) in a Sinclair front rest (with freely-rotating Rodzilla top) and Protektor rear bag. Barrel regularly cleaned with dry patches but not during this sequence. Muzzle FPS across the 30 shots ranged from 766-775 with SD of 2.4; no correlation of vertical spread with FPS. Reservoir pressure fell steadily from about 195 to 156 bar across the 30 shot sequence.

Please note the y-axis wavy pattern of shots across the rows. I see this in a lot of my shooting with this gun, even when using different rests. Sometimes there is a waviness in the x-axis spreads but not in this sequence. Overall the spreads of POI to average POI are not that bad, particularly for row 14. But if I can find the cause of the waviness and address it, my scores might improve.

Do any of you see similar patterns in your shots?
Any ideas on what causes this?, e.g.,
- Is this simply randomness and I'm trying to discern a pattern out of random results?
- Is something happening to my hold and thus torque on the gun as I pan across the target? (Except this happens with lots of variations on holds)
- Is lead building up in the barrel and then periodically getting released?
- Is this the result of some sort of spiraling where the pellet reaches the target at different points of it spiral? (But the variations are mostly up-down without accompanying x-axis shifts I'd expect in a spiral)
- If barrel harmonics, why are the changes gradual from shot to shot?

15-shot shots.jpg
 
I’d say it’s a bit of an “optical illusion”

Your zero at 25 is 1/4” (or whatever the line below the 14 line measurement is) low. If you adjusted the zero, the spread would be above and below the intended aim point. 11 of those 14 shots on the first line were nearly the same elevation. Imagine those red circles down a 1/4” and your shots would appear much more consistent.
 
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Thanks DellaD. But I should have mentioned that the POA was set for other shooting conditions and I did not re-sight for this sequence. I realize that if I had sighted-in properly, the spreads would score about a 240 if extrapolated to 25 shots. So overall the results are good. I just think the wavy pattern suggests something that can be addressed to get even better results.
 
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Are you repeating everything exactly the same, as much as is humanly possible? Even paying attention to pulling trigger in between heartbeats? After 5 years of doing this it dawned on me TODAY that my accuracy was better if I lightened up on my cheek weld. My gun is accurate enough to, literally, put shots through the same hole time after time @ 50 yards if it's locked in position. Unfortunately I'm not as accurate 🤣 . That's all I've got, probably doesn't help much.
 
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I assume your sorting your pellets, both weight and glass rolling . Lubing once they are sorted should cut down leading. Check you mag to see if there are any lead fragments that might get stuck on your pellet head as it is pushed into your breach. How about the spring tension in your mags? Not making consistent cheek weld, trigger finger pull not constant with breathing and heart beat?
Just some ideas to look at and consider. Chances are it's you not your gun.
That's why I like a gun that can out shoot me, if there's a problem I know where to look, especially with a piece of equipment so well tuned.
 
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I think that is a non regulated gun, no? If so…the valve pulse will be different as you change input pressure….despite the velocity spread being good. Valve pulse has a lot to do with vertical.

I assume you are not resting the gun on the reservoir?

Try tethering the gun at a fixed pressure. This will rule out valve pulse difference. Try different tether input pressures.

If you don’t have a tether hose that’s very supple….just shoot a 25 shot group so you don’t have to move the gun. Try to arrange the hose as best you can to avoid any pulling in one direction or another from the hose. Best is a 4ft hose from air tanks plus. It’s the softest and most flexible hose I’ve ever seen.

Don’t waste your time sorting. Crummy pellets go all different directions. A good setup for 25m will shoot a 25 shot group inside the 9 ring (.315”) without breaking it with pellets straight from the tin….and not discernibly smaller with sorting.

Mike
 
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Thank you everyone. I appreciate the suggestions and will try them.
But Old Spook got me thinking about how to assess whether the waviness is random. I'm pretty low-skilled at statistical analysis but I tried a brute force technique... I used a random number generator to produce 52 15-shot patterns like the rows in my initial post; see attachment. 36 of those shot lines showed at least some waviness. So I suspect I can't rule out randomness as a potential explanation of the patterns I've regularly seen.
Sorry for sending out an SOS for something that is probably not a real issue.
But thanks for the inputs.
Random number generated shot patterns.jpg
 
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Shoot about six five shot groups graph the horizontal and vertical spreads. If there's anything there that you should notice it will probably show up with that number of samples. If you feel like you're seeing something after shooting six five shot groups but you're graphing doesn't convince you shoot another six and graph those on top of the first six by the time you've done that you'll know whether or not you've got any stringing which isn't caused by wind or some external Factor.

Regarding comments made about sorting pellets by rolling them on a table. We have already repeatedly dispelled the myth that does not improve groups. Of course it improves groups how could it not? But that is another discussion that we don't really need to have here.

Pellet Rolling ~~ Part 1.

There are five parts to this series. They will be linked here at the end of today's editing session: PART ONE PART TWO PART THREE PART FOUR PART FIVE Rifle: HW-98, 0.20 Optic: SWFA 10 Test Pellet: H&N Baracuda at about 688 fps Distance: 25 meters. First I shot five different...
www.airgunnation.com www.airgunnation.com
 
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Again thanks for your guidance. In a recent 25m BR match, I shot a 740 score. One card was a 249. Your inputs helped me achieve this.
Specifically:
- Thomasair said to not waste time sorting pellets. I now agree after comparing average distances from mean POI of two batches of .177 JSB 13.4 MRDs In one case, a batch that always shoots well averaged 2.6 mm from mean POI for >120 shots, despite shooting unsorted pellets that ranged greatly in physical features. Another lot averaged 3.0 mm (> 120 shots) despite selecting only pellets with the same weight, head size, yrrah roll, good head/skirt appearance, and good center concentricity.
- Thomasair 's suggestion to shoot 25 shots at the 25m BR target looks like a great way to test lots. The attached picture shows the highly sorted pellets and the less-than-stellar results helped convince me to not use them in the match. I used the good lot (but did not shoot the 25-shot test because of limited numbers of those pellets. In the future, I'll shoot 2 or more of these 25-shot tests to screen new lots.
- Old spook 's and Ghostranger169 's recommendation of shooting a series of 5-shot groups helped me assess whether my POI changes as I pan across the target. The attached picture suggests some y-axis decline as I pan left to right. Shooting a couple of 25m cards and measuring shot by shot confirmed this. Those were all shot in typewriter fashion, i.e., left to right, then move down a row and start at left again. For my match I decided to switch to a different panning approach, i.e., started at bottom left and moved up, over one, then down, over one, then up, etc.
25 shot groups with JSB 13.4 MRD.jpg
5-shot groups with 13.4 MRDs.jpg
 
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