Benchmark muzzle velocity variation with ambient temperature

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rugbydave

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I decided to try and work up some new more in depth ballistics tables for my rifle and was using a program that took into account muzzle velocity variations as a factor of ambient/ammo temperature. Ideally you'd have a chronied speed for your load at all temps but it had a generic curve that was supposed to be quite applicable to most rounds. Anyways, I normally just chrony my loads during development and never really bother after as it's a pain to set up. My load, 40.3 grs of Benchmark pushing a .308 Hornady 168gr HPBT in Fed cases with CCI 200 primers gave me a MV of 2629fps @ 25*C this summer. I'm quite happy with is as I've been regularly shooting sub-MOA groups of 5 (0.82-0.72moa) at 200yds with it in my tweaked M14. Anyways, I decided I would try out these new ballistics charts that I had made up yesterday but that I would also chrony my load to have more data points. Turns out I got a MV of 2625fps @ -5*C yesterday. I know that Hodgdon advertises Benchmark (and all the "extreme") powders as being quite temperature insensitive but has anyone else had any experiences like this? That seems ridiculously stable? Looks like I'll be chronying my loads more often now but won't be getting any data much above 0*C for a few months, ha. Wish I had the coin for a Labradar...
 
Could be stable, or could be lighting changes from summer giving a higher FPS than what your rifle actually is doing. Chrony's can be very finicky depending on sun angle, cloud cover, or a combonation of
 
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