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Posted: 6/9/2013 2:51:20 AM EDT
| When loading for a match to be held in a high density altitude/low humidity area I am questioning my theory due to the conditions that I am loading in. Where I`m loading it is is 100% humidity and we are seeing a drop off in fps using our standard load ( 42 gr H4350 ) which generates ( normally) 2800 fps. Yesterday we were testing and the "normal" load was only giving us 2750 fps. What can I expect at the higher altitude/low humidity conditions that we will see at the match? |
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Quoted:
I'd expect a higher velocity due to lower air density. Of course, your FPS reduction could be due to conditions inside the case when loaded. Keep in mind, however, that you're only looking at a 1.8% reduction from 2750 to 2800. We did`nt see any poi differential at the range yesterday so it isn`t TOO big of a deal. What worries me is that the load we`ve been using has been incredibly accurate for a good while now. My son, whom I load for, is questioning the probable change. IMHO the humidity here is the issue NOT the powder charge. I loaded a few at 42.3 gr and only picked up a bit of velocity. |
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I shoot a 6.5 Creedmoor out to as much as 1100 yds..
My load is 41.2 H4350 140 AMAX which gets me ~2800 fps out of a 28 inch Criterion 2750 fps @ 500yrds 140 AMAX drops 49" 2800 fps drops 47" ~2" difference in POI 2750 fps @ 1000yrds 140 AMAX drops 311" 2800 fps drops 298" ~13" difference in POI Depending on the distances and size of target you are shooting 50fps can make a big difference. Humidity makes basically no difference. Altitude has a big impact as does temp but to a lesser degree.. Chronographs are not as accurate as we would like them to be. My guess is your drop in fps is do to something like neck tension, case volume, primer selection, charge accuracy etc. |
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Humidity has very litttle to do with a change in muzzle velocity. More likely is temperature being the culprit. Your muzzle velocity is produce by a chemical reaction which is pretty much self-contained, not affected by humidity, but very dependent on temperature. For example, pistols can be fired under water (thank you, Myth Busters). This is the internal ballistics. Density altitude comes into play for the external ballistics. Air that is more dense affects the flight of a bullet more. So, a bullet travelling at sea level, with high humidity, and low temperature will drop more than it will at high altitude, low humidity, and high temperature. (Technically, it's not the altitude, it's the barometric pressure that affects density.) While the humidity does have an effect on the density altitude, it's not huge. Some quick calculations on a density altitude site shows the change from 0%RH to 100%RH might only be 400 ft, with other factors held constant. http://wahiduddin.net/calc/calc_da_rh.htm ETA: There is a good explanation of DA as it relates to long range shooting here: Look at the drop chart he has in the middle of the page |
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