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Discussion Starter · #1 · (Edited)
Took my new chrony out today (Competition Electrics ProChrono) and was testing a 270 Win load, Federals 150gr Fusion. This shoots best out of my Ruger American and I wanted to know the exact velocity so that I could better use my BDC reticle and estimate bullet drop. First shots were from about 10' away, perfectly parallel to the chrony, got velocities of; 2849, 2812, and 2836. With an advertised of 2850fps, this seemed perfectly acceptable. Then I changed my position so that I could shoot through the chrony and hit my target, creating an angle, and my velocities dropped into the low 2700's. I was like...Huh? It took me a second to realize what had happened.

SO, when I got home I grabbed my pistol crossbow and chronographed it. Ten shots, all as parallel and straight as I could get them; avg of 167 fps, with none of the shots leaving the range of 166-169 fps. So pretty consistent. I started shooting angled shots through the chronograph, mild to severe angles, with an average loss in feet per second of 6-10fps, depending on severity of the angle. That equates to about a 3.5-5.5% decrease in velocity that the chrony is reporting. So on a rifle that should be shooting 2800fps, a slightly angled shot through a chrony could reduce your reported velocity by about 100-150fps. While this isn't surprising or unexpected, it certainly illustrates how important it is to be very consisted with your chrony setup so that you don't inadvertently make the bullet travel longer through the sensors than it needs too. Conversely, if your chrony is angled up or down but your bullet is parallel to the ground, you will make the reported velocity faster.

Edit: I mispoke regarding my last sentences. A chrony that is not level and is angled would still create a situation where it falsely reports the bullets velocity as slower due to creating a longer distance between the sensors.
 

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Just for kicks...which velocity is correct...straight or angled?

It would seem reasonable to think that the longer the bullet is in the screen, the longer the unit has to calculate the speed, the more accurate it would be.

Honestly, I have never noticed that in all the years that I have chrono'd bullets...but, I typically just shoot straight through and don't shoot at a target.
 

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Good point! Never really thought about that. Thanks!
 

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What's the distance between the front and back screen?
 

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Discussion Starter · #6 ·
Just for kicks...which velocity is correct...straight or angled?

It would seem reasonable to think that the longer the bullet is in the screen, the longer the unit has to calculate the speed, the more accurate it would be.
Angled is completely inaccurate. The whole point is that the chronograph is based on a known distance between the two sensors. If you increase that distance because the bullet is coming through at an angle, then the it takes longer for the bullet to reach the second sensor, giving a reported velocity that is in fact slower than it actually is. Check out the image below from precisionrifleblog.com, and it better illustrates what happened to me earlier today.
effects-of-an-unlevel-chronograph1.jpg

What's the distance between the front and back screen?
I think all the complete (non folding) units are 12" between the sensors.
 

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Took my new chrony out today (Competition Electrics ProChrono) and was testing a 270 Win load, Federals 150gr Fusion. This shoots best out of my Ruger American and I wanted to know the exact velocity so that I could better use my BDC reticle and estimate bullet drop. First shots were from about 10' away, perfectly parallel to the chrony, got velocities of; 2849, 2812, and 2836. With an advertised of 2850fps, this seemed perfectly acceptable. Then I changed my position so that I could shoot through the chrony and hit my target, creating an angle, and my velocities dropped into the low 2700's. I was like...Huh? It took me a second to realize what had happened.

SO, when I got home I grabbed my pistol crossbow and chronographed it. Ten shots, all as parallel and straight as I could get them; avg of 167 fps, with none of the shots leaving the range of 166-169 fps. So pretty consistent. I started shooting angled shots through the chronograph, mild to severe angles, with an average loss in feet per second of 6-10fps, depending on severity of the angle. That equates to about a 3.5-5.5% decrease in velocity that the chrony is reporting. So on a rifle that should be shooting 2800fps, a slightly angled shot through a chrony could reduce your reported velocity by about 100-150fps. While this isn't surprising or unexpected, it certainly illustrates how important it is to be very consisted with your chrony setup so that you don't inadvertently make the bullet travel longer through the sensors than it needs too. Conversely, if your chrony is angled up or down but your bullet is parallel to the ground, you will make the reported velocity faster.

Edit: I mispoke regarding my last sentences. A chrony that is not level and is angled would still create a situation where it falsely reports the bullets velocity as slower due to creating a longer distance between the sensors.
We can't cheat physics no matter how hard we try.
However marketing people are really good at making some believe they can also politicians and reporters ignore it completely.
 

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Discussion Starter · #8 ·
We can't cheat physics no matter how hard we try.
However marketing people are really good at making some believe they can also politicians and reporters ignore it completely.
Right....

I just found the physics of this situation interesting. You certainly see a lot of informal chrony reports on places like YouTube where they may not realize how much a non-level chrony, or an angled shot can effect the velocity reading.
 
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