Official answer from Adam...Shotmarker odd question about ave. velocity

[r bose]

Shot the last 2 weekends at 600 yards using the same 2 rifles, loads, projectiles, etc. Everything the same

Weather conditions just about identical, hot and humid here in SE Louisiana, not more than a degree or two different. Not enough wind either weekend to be moving the targets around or disrupting any of the shotmarker readings.

We used 2 different Shotmarker systems independent of each other. 2 different 600yd ranges. SD was spot on over a 20 shot string for each of the shotmarkers BUT, the average velocity was almost 100FPS more on 1 system over the other.

How does that happen and has anyone else experienced this?

 

[Walt Krafft]

Happens all the time. The velocity is not calibrated to give an accurate speed.

 

[Steveb]

If you shot on 2 systems, did you use separate target boards? If you did, what is happening is the dimensions for the spacing of the sensors is slightly different between each target setup. I had exactly the same thing happen to me recently. Our match targets use machined aluminum frames where the sensors are all within 1mm in all dimensions while we use wooden frames for practice. The practice frame sensors are located sort of precise. The velocity difference is over 100 fps between the frames and it is repeatable.

 

[r bose]

If you shot on 2 systems, did you use separate target boards? If you did, what is happening is the dimensions for the spacing of the sensors is slightly different between each target setup. I have exactly the same thing happen to me recently. Our match targets use machine aluminum frames where the sensors are all within 1mm in all dimensions while we use wooden frames for practice. The practice frame sensors are located sort of precise. The velocity difference is over 100 fps between the frames and it is repeatable.

Thanks for the info. Isn't it a bit weird the system would show FPS at the target if it's not correct? Why bother?
I'd like to know if shooting at 600 yds, what is the recommended spacing of the sensors so that the speed in FPS is correct? Perhaps an email to Adam for answers.

 

[Steveb]

The velocity calculation at the target depends on the spacing of the sensors. Every setup displays the correct readings for that particular dimensional setup. If the dimension are different between frames you will get different readings. I have enough experience with the system now to trust the readings but I made sure our match target frames were exact.
There are no "correct" spacing numbers for any range and target frame setup. You have to measure the spacing on your setup and enter those numbers into the system. Our 500 M and 900 M frame are the same at 6' square while our 300 M use 3' and my 200 M uses a 2' square frame.

 

[Krogen]

The velocity calculation at the target depends on the spacing of the sensors. Every setup displays the correct readings for that particular dimensional setup. If the dimension are different between frames you will get different readings. I have enough experience with the system now to trust the readings but I made sure our match target frames were exact.
There are no "correct" spacing numbers for any range and target frame setup. You have to measure the spacing on your setup and enter those numbers into the system. Our 500 M and 900 M frame are the same at 6' square while our 300 M use 3' and my 200 M uses a 2' square frame.

I'm perplexed. I don't see how sensor placement affects velocity readings. Yes you have to enter the calibration dimensions into the setup, but this is to triangulate the shot's impact location not determine velocity.

There are two microphones in each corner sensor. The shock wave from the bullet crosses over those two sensors and calculates velocity. Microphone placement should have nothing to do with it since the dual mics in the sensor are fixed in place. Just like an optical chronograph where skyscreens have a fixed placement and velocity is determined by the time difference. One reason the Shotmarker velocity is perhaps no-so-accurate is the mics in a corner sensor are just inches apart in contrast to an optical chronograph.

 

[Big_Daddy]

Here's a novel idea, why not get in contact with Adam and have him address/explain this.

Just a thought....

 

[Walt Krafft]

It might be that the microphone holders are 3D printed plastic, and the distance between the two mics in the holder is not consistent enough to measure the velocity accurately. Good enough to triangulate the position.

 

[AWCMKEN]

I think that shot marker uses a standard speed of sound measurement when calculating velocity. In the real world this varies due to temperature, humidity and pressure.

 

[xr650rRider]

Adam has an account on here. Here is a link to an old thread when shotmarker first came out.
https://forum.accurateshooter.com/t...l-8-mic-e-target.3944458/page-2#post-37214144

 

[r bose]

I'm perplexed. I don't see how sensor placement affects velocity readings. Yes you have to enter the calibration dimensions into the setup, but this is to triangulate the shot's impact location not determine velocity.

There are two microphones in each corner sensor. The shock wave from the bullet crosses over those two sensors and calculates velocity. Microphone placement should have nothing to do with it since the dual mics in the sensor are fixed in place. Just like an optical chronograph where skyscreens have a fixed placement and velocity is determined by the time difference. One reason the Shotmarker velocity is perhaps no-so-accurate is the mics in a corner sensor are just inches apart in contrast to an optical chronograph.

This is what I was thinking also?

 

[r bose]

Adam has an account on here. Here is a link to an old thread when shotmarker first came out.
https://forum.accurateshooter.com/t...l-8-mic-e-target.3944458/page-2#post-37214144

Went back and read this 2018 posting and according to Adam velocity should be accurate on Shotmarker systems, at least that was his thought at first. He explained how he took velocity at the target and calculated muzzle velocity. I was wanting to input all variables to calculate bullet flight out to a mile. Can't do that if velocity isn't accurate at the shotmarker system. I have an email sent to Adam.

 

[r bose]

Hi Robert,

Thanks for reaching out. The accuracy of the velocity at the target is dependent on the sensors being parallel. If you have a well built frame with parallel sensors, the velocity will be exactly right within about 5 fps, on average. There is also some random shot to shot variation if it's windy.

Generally, if the frame is well built, the average of a few shots is pretty much correct, if you are looking for an average on-target velocity. That's because the average just depends on the sensors being parallel to each other which you'll get if you have 5-star shot quality as reported in the sensor monitor.

The random variation on each measurement depends on the wind mainly. If it's super calm and you have a smaller frame the velocity can be within about 5 fps. However if you have a larger frame in light winds it might be +/- 10-20 and it can be even 50 fps off per shot if it's really windy.

Feel free to share some photos of your setup if you like. Thanks,
Laura

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Thanks to Adam for responding so quickly!!!!!!!!
Robert

 

[Acyr]

One issue we have found here in Phoenix that affects the parallelism of the sensors is the brackets that hold the sensors swell over time. This prevents the sensor from sitting flush into the magnetic bracket. It is easily overlooked because the magnets are so strong.

 

[Jeff Porter]

Your angle to the target ( perpendicular being ideal) affects the result too. The microphone spacing is only fixed and calibrated if the bullet is going straight through the target. So not just square plane target face but square to firing point.