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Long Range Hunting & Shooting
Different State - POI Change....is this normal?
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<blockquote data-quote="greenejc" data-source="post: 1632494" data-attributes="member: 60453"><p>Okay, I just went to the FM23-40 Sniper Training manual to refresh my memory. With a 7.62X51 round traveling at about 2600fps and a BC of 530, which would be comparable to your BC, a change of 2500 feet in altitude results in a change of about .5moa in impact upward. A change of 20degrees in temperature is a change of 1moa with the colder temperature meaning a downward change(1moa low). You were shooting at 2500 feet higher altitude and 40 degree colder conditions. You've got a 250fps advantage in velocity with the same or very close to the same BC. The change in impact due to altitude would be about 1/2 to 3/4 moa, and due to the temperature, about 1 1/2 to 1 3/4 moa lower. So 1 inch low at 2500 feet and 40degrees colder is about right. The impact left was almost certainly either mirage or wind drift, which shows up with even a 5 mile per hour wind. And there's always wind in Kansas. A 10 mph wind at 2500 feet and 40 degrees temperature would have drifted your round around 0.6 inches at 100 yards, according to the JBM ballistics calculator. The Shooters Calculator gives a wind drift at that altitude and temperature for a 10mph cross wind of 0.59 inches. I think the difference is due to a combination of altitude and temperature, with a little crosswind thrown in.</p></blockquote><p></p>
[QUOTE="greenejc, post: 1632494, member: 60453"] Okay, I just went to the FM23-40 Sniper Training manual to refresh my memory. With a 7.62X51 round traveling at about 2600fps and a BC of 530, which would be comparable to your BC, a change of 2500 feet in altitude results in a change of about .5moa in impact upward. A change of 20degrees in temperature is a change of 1moa with the colder temperature meaning a downward change(1moa low). You were shooting at 2500 feet higher altitude and 40 degree colder conditions. You've got a 250fps advantage in velocity with the same or very close to the same BC. The change in impact due to altitude would be about 1/2 to 3/4 moa, and due to the temperature, about 1 1/2 to 1 3/4 moa lower. So 1 inch low at 2500 feet and 40degrees colder is about right. The impact left was almost certainly either mirage or wind drift, which shows up with even a 5 mile per hour wind. And there's always wind in Kansas. A 10 mph wind at 2500 feet and 40 degrees temperature would have drifted your round around 0.6 inches at 100 yards, according to the JBM ballistics calculator. The Shooters Calculator gives a wind drift at that altitude and temperature for a 10mph cross wind of 0.59 inches. I think the difference is due to a combination of altitude and temperature, with a little crosswind thrown in. [/QUOTE]
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Different State - POI Change....is this normal?
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