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There's an old saying in electrical engineering, "When you measure something with two different instruments, expect to get two different values." Fortunately, often the differences are small enough to not cause concern. Such is the case when I compared velocity measurements using LabRadar and MagnetoSpeed.

School closed yesterday at noon due to the risk of tornadic activity in the area, so I took off to the range. I wanted to measure the velocity of HPR 55 gr VMAX out of a Faxon 18" Gunner barrel (1:8). I started with the MagnetoSpeed and then switched to LabRadar. I should have done them both at the same time, but I had some reasons not to do that, that I don't need to get into here. So we are at a slight disadvantage of not only different instruments, but different ammo, different time, and different barrel temps. But it actually came out pretty close.

The MagnetoSpeed measured 13 shots and calculated an average velocity of 2629 fps with a SD of 27.4 fps. I measured 8 shots with the LabRadar and got an average velocity of 2601 fps with an SD of 30.2 fps.
 

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The OCD side of me is curious why you didn't run an equal number of rounds through each?
 

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The 1% variation between the two averages seems fine, but the standard deviations indicate a greater dispersion with the Lab Radar.

At my club's chrono station for the Level 2 & 3 matches we run two (conventional) chronos in series. The "downrange" chrono obviously should reflect a slower bullet speed, but that's masked by the repeatability of the units which seems to run about 1%. Any chance you can arrange the two chronos to measure the same shot?

Re the statement about two different instruments, in my circle the old saw is "never ask a man with two watches what time it is." Same deal!
 

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Discussion Starter · #4 ·
The OCD side of me is curious why you didn't run an equal number of rounds through each?
A daughter was cooking a ham for her father's birthday. He noticed she cut it in half before putting it in a big pot. The dad asked why she did that. She said that's the way mom always does it. So he asked his wife why she always cut the ham in half before putting it in the pot. She said because that's the way her mom always did it. So the dad calls the mom and asks her. She said I had to cut it in half to get it to fit in the small pot I had.

I only had 8 rounds in the box to shoot.
 

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The MagnetoSpeed measured 13 shots and calculated an average velocity of 2629 fps with a SD of 27.4 fps. I measured 8 shots with the LabRadar and got an average velocity of 2601 fps with an SD of 30.2 fps.
With those speeds, I would not think there would be enough difference there to be statistically meaningful
 

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Discussion Starter · #6 ·
The 1% variation between the two averages seems fine, but the standard deviations indicate a greater dispersion with the Lab Radar.

At my club's chrono station for the Level 2 & 3 matches we run two (conventional) chronos in series. The "downrange" chrono obviously should reflect a slower bullet speed, but that's masked by the repeatability of the units which seems to run about 1%. Any chance you can arrange the two chronos to measure the same shot?
SD can be tricky in that it is a calculated value based on actual data. We could have two groups, one with a large SD and one with a small SD and yet they could both have the same average velocity.

Since the SD is a measure of how far the data points are from the mean, two bullets could be separated from the mean equally but on opposite sides of the mean and the SD would be increased but the average would not be changed.
 

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Very interesting, but now yer messin' with my mind, Tangle.

I didn't want to know this for now I'll be fretful about Smitty's 1% and deviations that are anything but standard, what with that dispersion he mentions.

"So we are at a slight disadvantage of not only different instruments, but different ammo, different time, and different barrel temps. But it actually came out pretty close."

Considering the above variances, this is like nothing at all. That is to say:

Close enough for government work.
Close enough for the girls I go with.
Close enough for jazz.
Close enough for horseshoes.
Close enough for hand grenades.
 

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Very interesting, but now yer messin' with my mind, Tangle.

I didn't want to know this for now I'll be fretful about Smitty's 1% and deviations that are anything but standard, what with that dispersion he mentions.

"So we are at a slight disadvantage of not only different instruments, but different ammo, different time, and different barrel temps. But it actually came out pretty close."

Considering the above variances, this is like nothing at all. That is to say:

Close enough for government work.
Close enough for the girls I go with.
Close enough for jazz.
Close enough for horseshoes.
Close enough for hand grenades.
LOL, well I'm just glad somebody besides me is fretting over this stuff! :tongue:

Let's see what today's simultaneous measurements yield :blink:
 
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