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Need help, Sudden elevation drop at 300

 
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  #8  
Old 11-21-2013, 05:11 PM
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Location: Yellowknife, NT Canada
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Re: Need help, Sudden elevation drop at 300

I shoot in the Arctic at temps varying from -30 to 80 deg F, so have dealt with temp variation quite a bit, and how it affects the muzzle vel. A general rule of thumb (it has worked fairly well for my shooting) is a change of 3 fps for every degree of change. All depends on powder type etc. As previously mentioned, this alone prob wouldn't account for the entire change in POI, but may be a contributing factor.

Note: The only chrono that works reliably for the colder temps up here is the Magneto Speed. I take the LCD controller off between strings and warm it up in an inside pocket.
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  #9  
Old 11-21-2013, 05:20 PM
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Re: Need help, Sudden elevation drop at 300

All right, I got un-lazy and ran the numbers. The change in POI at 300m shooting a 215 Berger at 3000 fps at a temp of 70F and shooting the same bullet at 2900 fps at 40F is approx 2 in. This takes into account both the drop of approx 100 fps in muzzle vel due to the change in temp, and the increase in air density due to the colder temp. Given that that's just a rule of thumb calculation, your change in POI could be as a result of the temp change, although it's likely your site picture due to cloudy vs sunny etc could have had some impact as well.
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Old 11-21-2013, 05:59 PM
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Re: Need help, Sudden elevation drop at 300

I'm curious as to what powders you've seen 3 FPS per degree of temp changes. I cannot Phathom 90 FPS changes from 30 degrees. I'm also fortunate to live where I get to test loads in sub zero temps and compare them against loads that are 80-100 degrees. I've actually compared some as cold as -20 against 90 degrees. The results have been quite shocking. Much less than I ever would have thought. At least with RL powders and H extreme powders. Never tested ball powders that way. Double base powders have been more sensitive. Like .65 FPS per degree. To me, that's quite a bit. I'm always hearing how bad RL powders are and I still don't get it. R15, 19, 22 and 25 for me have shown to be very minimal. As good or better than the H extreme powders. R17 is a bit different. But then again, it's a true double base powder.
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  #11  
Old 11-21-2013, 06:38 PM
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Re: Need help, Sudden elevation drop at 300

Quote:
Originally Posted by Michael Eichele View Post
I'm curious as to what powders you've seen 3 FPS per degree of temp changes. I cannot Phathom 90 FPS changes from 30 degrees. I'm also fortunate to live where I get to test loads in sub zero temps and compare them against loads that are 80-100 degrees. I've actually compared some as cold as -20 against 90 degrees. The results have been quite shocking. Much less than I ever would have thought. At least with RL powders and H extreme powders. Never tested ball powders that way. Double base powders have been more sensitive. Like .65 FPS per degree. To me, that's quite a bit. I'm always hearing how bad RL powders are and I still don't get it. R15, 19, 22 and 25 for me have shown to be very minimal. As good or better than the H extreme powders. R17 is a bit different. But then again, it's a true double base powder.
Interesting. The only two calibres and powders I've worked with a lot up here are Varget in 308 and H4831SC in 300 Win Mag. I've always been a little leary of my velocities as I didn't trust my chrony that much, but my results have been fairly consistent. I just rxd the Magneto Speed and am working up a load with 215 Bergers and Retumbo (and H1000 when it comes in) in my 300 Win Mag. Last year, I had to tweak my 300 WM load significantly (3-4 gr) after my winter work up period as it was too hot in higher temps. Standard hunting temp around here is about 37F, so I'll fine tune things in the spring when we cross that threshold again. It's going to be a loooong winter. Always is up here.
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  #12  
Old 11-21-2013, 09:59 PM
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Re: Need help, Sudden elevation drop at 300

When on the 1000 yard line the difference when a cloud comes over to full sun is usually 3 to 4 inches. Dark is low sun is high. I doubt this alone caused that much at 300. Now if you don't take the parallax out of your scope that can affect it a lot. You would be surprised how many guys on the firing line at a match don't check or know how to check for parallax. Mirage is another vertical killer. Matt
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  #13  
Old 11-21-2013, 10:54 PM
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Re: Need help, Sudden elevation drop at 300

you all probably don't need my 2 cents worth, as you have a very informed group here, but I have seen 2-3 fps per degree of variance above or below 72 degrees F using real slow ball powders like H870, AA8700 etc ,,,,,,

in the big Lazz/Wby/RUM mags, using reloader 19, reloader 15 & VVN560, you are looking at 1 to 1.5 fps per degree, above or below 72F ,,,,,,,

the reason that 72 degrees F is important, is that is the temperature that most SAMMI pressure/velocity standards are established at ,,,, to be consistent from caliber to caliber ,,,,,
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