Accuracy, Spread and Standard Deviation

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BrocStar

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Sep 4, 2021
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California, United States
Hello all, Sorry for my rant ahead of time. 

So I have a question. When looking at consistency with FPS or accracy at a certain distance what is more important. A consistent spread under 12 fps and low standard deviation? or an accurate consistent shot at a certain distance?



The reason I ask is I seem to be getting really fixated on fps spread and standard deviation. On my Impact M3 I was shooting hole in hole at 30 and 50 yards . But my spread was way off. like 30-50 fps difference occasionally . So I started to change my tune. I got the spread and fps within 12-14 and standard deviation to about 3.5 but my shots were off. I am the type of person that will fixate on something and can't sleep until I get it right. My nick name given by my mother is "one track" as in one track mind... Its a flaw and a gift I guess because I do not quit until I get it right and get what I want. Second nick name Badger . Also given by my mom. Hah. Well If I have some type of direction to know I am in the right direction or that 10-14 spread is completly normal I move on to my next task.

What a reasonable spread and STD? Something that's not perfect but okay to work with? If I am shooting hole in hole at 50 yards and I have a spread of 24 is that okay? I only use unsorted JSB 34 pellets.

Thank you for all the help.

Broc 

Thanks
 
Broc,

My advice would be:

If your goal is accuracy, let ACCURACY be your guide(btw, don't mess with a gun that is stacking pellets, until it stops stacking them).

If looking at a string of measured fps numbers(which may or may not actually be 100% correct) is your goal, then let that be your guide. 

P.S.- Don't fix it if it ain't broke LOL
 
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Broc,

I've got to agree with the others. My Gen One Umarex Gauntlet .25, shooting JSB 33.95 grains at a modest 739 FPS, has a SD of 7.62 and an ES of 24.9. The accuracy is phenomenal, first to last shot, and every time I point it at a crow and pull the trigger, one less crow. If the gun wasn't 47 inches long and over 10 pounds with a scope, I'd have never had the need for another PCP. WM
 
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I test my chrony frequently with a single stroke pneumatic rifle that I know is consistant. If I get weird readings I clean the screens,check the lighting,and replace the battery. I also placed clear 3M packing tape over the screens so cleaning is as easy as wiping the tape with damp cloth.

Getting the wrong data from your chrony can be frustrating.
 
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Bob Sterne, who mostly posts at www.GatewayToAirguns.org (GTA), has written an article about this that has opened my eyes to understand our shooting hobby at a whole new level — in just one simple chart Bob put four separate factors together that we usually don't see together.



See the chart below for yourself, it describes how much the following "errors" each affect the group size at different ranges:

• Extreme spread ES (muzzle velocity variation)

• Wind estimation error

• Ranging error

• Barrel precision (group size the gun is capable of)


Matthias




Ballistics. Impact on Group Size Of Wind Estimation Error MV Variation ES Ranging Error ...jpg




Link to the article:

https://hardairmagazine.com/ham-columns/how-important-is-extreme-spread-to-accuracy/




 
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Depends on the intended use of the gun.

As an example, "regular" field target does not need tighter than about a 30fps extreme spread to shoot a perfect score. But that's a 10-55 yard game not really a game of supreme precision. 

From posts on the airgun forums, it seems that most airgunners are much more fixated on consistency ("single digit extreme spreads") than required for the types of shooting we do. 
 
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6/30/22

I am also having ES and SD Dev problems with my FX impact MK2. Below is representative Chrony results. Same settings with 34.1 gave ES 35 and SD of 9. In Decemberwith a .22 caliber barrel I was able to obtain a SD of 1.7, not any longer. I have adusted velocity up and down with hammer spring adjuster wheel and valve tuning knob, no improvements at any velocity. I am convinced something internally is to blame.

Shot Count 20

Hi 951

Low 926

Avg 940

Spread 25

STD dev 7.0

Hammer 18.0

Reg 125bar

Knob Adjuster 11.4mm

Caliber .25

Pellet FX 25.4 grain
 
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I think most of us are too easily bothered by ES (the old "too much information" problem). We have the technology, so we use it, but don't really know how to interpret it. I recently shot a long chrono string through my HW100 that I tuned down to 11.5 fpe, to determine the approximate reg set point, as I didn't use a test gauge in my tuning. The cylinder was about half charged, and I shot something over 80 shots. Until I was clearly off the reg, the ES was mostly very consistent, within 5 fps. But during those 80 shots, 3 or 4 were real outliers, 15-20 fps off the average. I have no explanation, but I suspect the pellet. I was doing no selection, since I wasn't shooting a target, and didn't care about accuracy. Some of the pellets I loaded were visibly imperfect, so I suspect the obturation in the bore, and thus the air seal, was not 100% with some of those pellets. It's possible, of course, that the reg could be approaching the point of needing a rebuild, but I doubt it. As the article in the link above demonstrates, ES of up to around 20 fps does not result in an accuracy problem at the distances and targets most of us shoot. I have suspected as much for a long time, and I rarely use the chrono now except for tuning. Unless one is engaged in a high level precision accuracy competition, ES should probably be very low on our list of concerns.
Ed
 
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JungleShooter said:
Bob Sterne, who mostly posts at www.GatewayToAirguns.org (GTA), has written an article about this that has opened my eyes to understand our shooting hobby at a whole new level — in just one simple chart Bob put four separate factors together that we usually don't see together.



See the chart below for yourself, it describes how much the following "errors" each affect the group size at different ranges:

• Extreme spread ES (muzzle velocity variation)

• Wind estimation error

• Ranging error

• Barrel precision (group size the gun is capable of)


Matthias




View attachment 207981



Link to the article:

https://hardairmagazine.com/ham-columns/how-important-is-extreme-spread-to-accuracy/
Click to expand...
Great article! I’m like broc, but maybe I should give it a rest!
 
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I just explained all this to a member today. Currently, both my 22 and 30 cal mk2 impacts are spot on accurate, with the 30 cal having a 16 FPS spread and o believe the 22 at around 12-14.

Three months ago I posted two shot strings out of my 30 cal- 92 straight shots with a 14 FPS spread and the second string was 34 shots with an ES of 2 FPS, and the accuracy wasn’t there. At best it was a 3/4”-1” group at 50 yards.

Now at 50 I feel like so I’m so in control of the shot, like I know for sure if follow good shooting technique it’ll go hole in hole at 50. It’s been a lot of work getting it to that point, but yeah, as long as I hover below a 20 FPS extreme spread and the accuracy rules, I’m good with it
 
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Im getting under a half inch at 50 yards and hitting eggs at 200 yards with my M3 and i had the same problem with a crazy spread, until i deleted my front reg. Now it takes less reg pressure to achieve the same velocity i was getting my reg set 20 bar higher than it is now. If your going for a power tune i would delete the front reg but if your shooting pellets then you need to find the right balance of hammer valve and reg. I get consistent 3-5fps spreads with my M3 .25.
Screenshot_20230528_171001_FX Radar.jpg
 
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