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....