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Discussion Starter #1 (Edited)
This Sunday I went to try out some 95gr TTXS 's loaded with 27.5gr of H4198. I ran 5 shots through the chronograph. The load produced a solid 2811fps average. Using Strelok I figured for a 200 yard zero. Which would put me at about 1.39" high at 100 yards. So that is where I dialed it in at. The weather was 80 degrees with 89% humidity and pressure was 29.80. Today I went back with the same loads and my POI changed by 1.6" lower than Sunday. The only thing that changed was the weather. Today it was 75 degrees with 45% humidity and the pressure was 29.9. It seems to me that humidity played a big roll in the POI for this weapon/load combo. Just to add I fired multiple 5 shot groups on each day and on each day they repeated for that day. You got to love the weather here in MN. One day it's worse than living in a rainforest. The next it is the most beautiful early fall day that there ever was.:a08:
 

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Ask a pilot about density altitude. ..........

Humidity is a killer . I've had the same thing come up with a 45 Colts carb working up a load at home and hunting hogs in south east Texas . 4100 ft average humidity 22% vs 80ft (maybe) and 70% . The end result ? 1 inch drop in poi @50yd and my full tilt Ruger level hog loads tumbled and I had to drop back to a deference Colts load with the huge nose SWC . Our live reading is generally down around 27.25-8 in hg vs the 29.7-9 at sea level . The speed of sound of course plays a big roll in that particular case .

Hard lesson learned there . Different load and bullet this year 1300 fps gets me completely out of the sonic transition range out to 100 yd or so . The Keith style SWC is vastly more stable than the Lee 452-252 when super sonic . .......rambling.

High pressure wet air vs dryer but higher pressure air and warmer thinner air vs cooler denser air . It is a complex equation that I can't do off the top (it's been too long) that will give exact comparisons of air density with changes in humidity, pressure and temperature. You would be surprised by how big a small change will be .
 

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Many powders are very temperature sensitive. I don't remember if H4198 is one of those but it could just be the temperature affecting your powder.
 

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Discussion Starter #4
Many powders are very temperature sensitive. I don't remember if H4198 is one of those but it could just be the temperature affecting your powder.
H4198 is a extreme powder and is pretty insensitive to temp. I think my POI was changed do to the extreme humidity and air density change. I have just never noticed it as bad as the what I came across this last week.
 
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