SD increase at range?

[DRNewcomb]

I noticed something yesterday and I don't know if this is real or an artifact of the Shot Marker electronic target system. When I tested my .308 loads through my CED M2 chronograph, I got a SD of 9.2 at the muzzle. At 600 yards yesterday the Shot Marker electronic target system gave SDs for 20+ round strings of 12.7, 17.1 & 18.6.

This got me thinking about the possible causes for the different SDs:
1. Since they were separate sample sets, loaded on different days and shot under different conditions, the SDs were actually different.
2. The Shot Marker system isn't really accurate for measuring SD and those numbers should only be used for entertainment value.
3. There are really things that happen between the muzzle and the target that act to increase the SD of rounds at 600 yards.

I was just wondering if anyone had studied this subject, for instance, by comparing statistics of bullets as they leave the muzzle and as they enter the target? Has anyone compared the velocity statistics provided by an electronic target system with those provided by a chronograph at the target? Just thinking out loud.

 

[CharlieNC]

Not statistically different, keep shooting!

 

[Ned Ludd]

The accuracy/precision of velocity measurement at the target face is an additional source of potential error that could be significant, as you noted. My understanding is that the reliability of velocity data generated at the target face by e-targets is not in the same league as that collected at/near the muzzle with a good chronograph. Did you actually compare SD values from the same shot strings chrono'd simultaneously at both the muzzle and the target face (i.e. compare data collected simultaneously with both the LabRadar and the ShotMarker)? If, in a simultaneous velocity determination, the LabRadar was giving much lower SDs than the ShotMarker, that tells you that the increase in SD values at the target face likely had more to do with the external conditions and/or reliability of the measurement at the target face; at least, it would suggest that the variance was generated by some mechanism occurring between the muzzle and the target face (i.e. after the LabRadar).

 

[chromatica]

I was shooting a mid-range F-Class match a few months ago and noticed just the situation you described. I was using RL-17 which is not known for being temperature stable. The extreme spread at the Shot Marker target was around 90 fps. If I looked at the min and max values and worked backwards, the ES at the muzzle would have needed to be around 125 fps. I didn't think it was likely that my muzzle velocities had that kind of spread, even with RL-17!

 

[David Christian]

Shotmarker speed data is not precise on its own. Add a gentle breeze that moves the backer board by only 1/4” and your numbers are blown wide open.

 

[AJC]

The measure at the target face will include any differences in the bullet as well as the charge. I would expect them to always be higher than directly in front of the gun. The lab radar if you have one may help show the difference. The resolution accuracy of all equipment will also need consideration.

 

[RegionRat]

The Shotmarker gives a velocity at the target for two reasons.

One. It was easy for Adam to show it as a side benefit of the way the system works in order to show the impact position, so it wasn't extra work. And, two, it is useful even if it is a rough number.

What we learned during Shotmarker Beta testing at BRRC, was mentioned above by @David Christian , in that if the transducers are being buffeted by the wind, the uncertainty in the velocity and position measurement goes up. When the transducers are on something more rigid, the data is noticeably more quiet.

As for other reasons a velocity stat is different down range than it is at the muzzle, those effects are also real and variable.

Atmosphere and winds, the actual BC variability of the particular bullet and barrel and twist combination, can all cause a real downrange velocity differences in addition to those at the muzzle. These are real effects we have measured with Doppler Radars that can track bullets way down range on their trajectory.

On a calm day, you will see those Shotmarker numbers come closer to the muzzle numbers, on a bad wind day, they can diverge if the target carriers are unsteady.

 

[rijndael]

Wouldn’t bullet to bullet BC variations also be a factor at distance?

 

[RegionRat]

Yes, that is the point of mentioning it. Not every bullet, twist, and bbl combination does the same at distance even if you had a perfect zero ES/SD at the muzzle in calm air.

 

[DRNewcomb]

On a calm day, you will see those Shotmarker numbers come closer to the muzzle numbers, on a bad wind day, they can diverge if the target carriers are unsteady.

So, if the conditions at a match start off relatively calm and the wind picks up as the match progresses, one can expect to see the reported SD for successive strings increase?

 

[JimSC]

The elephant in the room for me is the question "How were the scores?" Was the 12.1 a higher or lower score than the 18.6?

 

[RegionRat]

So, if the conditions at a match start off relatively calm and the wind picks up as the match progresses, one can expect to see the reported SD for successive strings increase?

Depending on the how the transducers are mounted and how flaky the target carriers are, the downrange SD/ES numbers increase because the SM is shaking, and also because the real dispersion in the bullet's performance is affected.

 

[rijndael]

Yes, that is the point of mentioning it.

Got it. Your post wasn't up when I was typing mine.

 

[DRNewcomb]

The elephant in the room for me is the question "How were the scores?" Was the 12.1 a higher or lower score than the 18.6?

Actually, my scores improved as the SD increased, albeit slightly. The two other shooters on my target were shooting F-O (I was F-TR). They both had much higher SDs and got much better scores than I did. (e.g. 199-11X vice my 189-191) Of course, conditions at the match started out somewhat foggy, clearing gradually and my last string was fired on a different target from the first two, so who knows?

 

[DRNewcomb]

Depending on the how the transducers are mounted and how flaky the target carriers are, the downrange SD/ES numbers increase because the SM is shaking, and also because the real dispersion in the bullet's performance is affected.

I find this a very interesting subject. I never considered the possibility that the quoted BC is just an average and that the BC of bullets in a batch has its own SD. When you think about it, it's intuitive but you have to think about it first.

 

[AJC]

I find this a very interesting subject. I never considered the possibility that the quoted BC is just an average and that the BC of bullets in a batch has its own SD. When you think about it, it's intuitive but you have to think about it first.

Not only does bc vary bullet to bullet, bc changes on the same bullet on different parts of its path. The situation is continually dynamic and I'm amazed we do as well as we do.

 

[ebb]

Are these small primer cases?

 

[DRNewcomb]

Are these small primer cases?

In my case, yes. Lapua Palma.

 

[JimSC]

Actually, my scores improved as the SD increased, albeit slightly. The two other shooters on my target were shooting F-O (I was F-TR). They both had much higher SDs and got much better scores than I did. (e.g. 199-11X vice my 189-191) Of course, conditions at the match started out somewhat foggy, clearing gradually and my last string was fired on a different target from the first two, so who knows?

there are so many variables involved the a main one of course being the person pulling the trigger. At time I wonder how much effect the minutia really has. Still we have to control the things we can

 

[DRNewcomb]

there are so many variables involved the a main one of course being the person pulling the trigger. At time I wonder how much effect the minutia really has. Still we have to control the things we can

Yeah. My calculator tells me that, for my load, ±20 fps is less than 2" at 600 yds.