Velocity and temperature

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Alpheus

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So here's the story. Last Saturday I was doing some load development, shooting groups for size and velocities. Temperature was around 13-14 C and cloudy. In my 308, 175gr OTM with 43.4gr N140 clocked in at around 2650FPS over my Beta chrony, SD single digits and 1/2MOA. Very nice load. Also tried some 208gr AMAX, gave me 2410FPS and 2/3MOA with 41.8gr of N150. My 6mmBR 105gr VLDs with 30.1gr Varget were getting a leisurely 2880, and once the barrel was re-coppered after cleaning, gave me a very nice 0.24MOA group with ES in the single digits.

On Sunday, myself and a few guys head out to our long range spot and set up some gongs at 546yds. Temperature is now 24-25C, very sunny. Break out the chrony, and find my dope. My 175gr was suddenly going 2960!, with slight ejector marks and only dropping 11MOA. According to the Applied Ballistic calculator, that drop should be achieved with a velocity of around 2850. Even that is a hell of an increase over 2650, let alone what my chrony is saying. I still had a few rounds left, so I pulled one and checked the powder. Spot-on 43.4gr

I then tried my 105VLDs, and the chrony says they were zipping out at 3080FPS, still accurate and low ES. Calculator says for the drop of 10.5MOA, they should be moving at 2900FPS Again, chrony gives way above what the calculator gives for the observed drop.

Then I tried the AMAXs again, nervously. Chrony decided to start giving errors, and the calculator said around 2460 for my drop of 14.5MOA, which is quite reasonable.

Is N140 just that variable with temperature? 300+FPS increase with just 10C temp increase!?! I place my chrony 10 steps from the muzzle every time, so it can't be a difference there. Used the sky screens both times. Is my Chrony just junk?
 
N140 is nowhere near that temperature sensitive.

What time of day were you shooting on Sunday, i.e. what was the angle of the sun, on this very sunny day? Was the sunlight shining directly on the light sensors, even though your skyscreens were directly above?

Chronys are cheap (nothing wrong with that, they are the chronos I have too); part of this is that you need to know how to work with (and sometimes work around) their limitations. One of the trickiest parts of designing an optical chronograph is to make your light sensor (and the associated electronics behind it) work across a very broad range of light intensities. Bright sunny days with clear skies are the most difficult conditions (overcast days are the easiest). On very bright clear days it can be necessary to use the skyscreens (like you did), and sometimes even then you'll get suspicious readings - sometimes I have found that putting something between the sun and the chrony, so a shadow is cast over the light sensor areas, can get things working properly on this sort of day (at some ranges this is straightforward, at others it can be impossible).
 
It was around midday, so the sun was high but not nearly close to being overhead. Sunlight could have certainly been falling directly into the sensors. I'll look into making some kind of shades for it, maybe some foam board on the diffuser posts or something.... I'll also try replacing the battery, you never know.

I'll be on the range again this Saturday, and the forecast is pretty similar, so we shall see what happens.
 
Sounds like a chrony issue to me as well. I get about 1.5 fps/degree f change (different for different temp ranges and rifles). Switched to the Magneto Speed about 8 months ago after 2 x unsat chronys and am very happy. No errors so far.
 
Welcome to Chronys, I've been preaching this for years !!! The last one I tried did exactly what you are describing on 2 consecutive days, same temps and altitudes etc, but over 200 fps difference with the same rifle and same load, just different light conditions. I'll never use or trust another Chrony as long as I'm still shooting. Those kind of differences are totally unacceptable and one would be better off without a Chronograph than with a Chrony POS !!!
 
There is a Steinart (sp?) Acoustic chrono on the EE I just saw. I've been looking into one of these myself. They seem like they are waaayyy easier to deal with. That being said, I haven't used any sort of chrono, but lurk on the forums and do all the research I can before I ever buy anything.
 
Don't know about a "bank breaker" but I have 2 Oehler 35Ps and they never seem to lie to me and I can check one against the other. Sometimes it's just better to save your money until you can afford the best, and chronographs seem to be one such item. Some folks say the Oehlers are too much money but in reality they are cheaper than even a mediocre rifle or decent scope, (around 700 bucks) and are invaluable when working up loads.
 
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I get the odd questionable reading on my chrony but you have to know when to write it off.

The people above covered the issues. The only way around it I think is the magnetic versions.

If you feel safe to continue shooting, as you did, ballistics don't lie. Confirm velocity via drop. It will error out the chrono on those bad days.
 
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