Chronograph issues?

davkrat

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Sep 18, 2006
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The Motherlode
I have a basic Shooting Chrony that has given me some strange readings lately. My load in the 7mm Rem Mag is 69.0 grains of H1000 and the 150 ETips. I regularly get an average velocity of 3131 fps and SD of 15. The last time I went out the exact same load averaged 2907 with an SD of 17 for 10 shots. My barrel does tend to copper foul with the ETips and I regularly give it a thorough cleaning. Anything that could cause an extreme drop of 200+ fps in velocity other than a faulty chronograph? These results were only a few weeks apart in nearly identical conditions, around 70 degrees ambient so no extreme temp changes. The fact it was consistently so inconsistent has me scratching my head.

I am shooting a pretty much maximum load out of a 26" barrel so I do not think the higher velocity is outside the realm of possibility of the 7mm Rem Mag and the drops match up very well at that speed.
 

ShtrRdy

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Jan 14, 2012
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High Plains
Was there a difference in sunlight? Was it cloudy one day and clear the other? Was it a different time of day?
 

Browninglover1

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Jan 10, 2011
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Northern Utah
Inconsistent readings with my old chrono is what caused me to buy a Magnetospeed. Now I never have to wonder if it's right.

Only other thing I can think of that might cause your problem is if your scale went out of calibration and you're loading less powder.
 

Lefty7mmstw

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May 13, 2012
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Dakota del norte
likely-- screwy chrono...

Unlikely but possible-- if you are using a different lot of powder you could be getting slower readings-- usually not that much though...
 

Jokostel

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Dec 1, 2011
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Bismarck,ND
With that powder charge, I doubt that you'd be seeing any greater than 3000 fps, even with the ETip....
However, every rifle is of it's own law... I have had 300 Win Mags that defy explanation as well.

However what your seeing is most likely a chrony issue.
New battery? Clean the Screens? Overall... find another chrony to test and confirm....

All your other variables you seem to have accounted for.
 

scf

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Jan 15, 2013
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171
Location
Western Kentucky
I was checking velocities through a Shooting Chrony one evening. Came back next day and velocity shown was 900 fps slower. Was my last day with a Shooting Chrony. I've had good luck with the PVM 21 (but too pricey) and the Magnetospeed.
 

royinidaho

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Jan 20, 2004
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8,950
Location
Blackfoot, Idaho
I'd suspect chrony setup.

When sky is cloud covered (way more white than blue) I don't use the white plastic reflectors.

When sky is blue and bright ya gotta use 'em.....

When a cloud passes over, with the shot, as compared to no cloud, velocity reading changes.

I would also be wise to ensure the chrony is consistently set up parallel with the line of sight of the bore. I noticed "somewhat" better consistency when I gave this more consideration.

I'm not too particular about session to session consistency but expect there to be a fairly close relationship between velocities during different shooting sessions.

Someone ought to do some calculations of how much difference the distance across the chrony makes with different tilt angles. Just wondering... But not motivated enough to do it myself.:rolleyes:
 
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