BigJohnH
Well-Known Member
After calculating what you said in your first post the air temp and altitude had no effect on the bullet at 100yds, at 1000yds yes.
Different State - POI Change....is this normal?
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SavageHunter11
Well-Known Member
Difference in altitude and humidity are probably the biggest culprits but temperature would be a factor too I'd imagine. You also have to remember while your cartridge powder is very temp stable everything else is affected by temp too. Barrels shoot different as they heat up so a 40 degree difference in barrel, case, etc temp could affect it too.
It's probably due to shipping your rifle. The environmental differences would be minimal. Also look into the angle you are zeroing when you zero, it will help you see how the incline effects your shot placement. When I shoot bulls switching from top left target to bottom right always shifts POI due to differences in angles and how that effects your rifle platform. I see a very small difference at 50 yards and at 100 yards it would be a much bigger difference. 1x.5" is probably normal considering you changed your sight in environment IMHO: so it may not be the scope. Keep in mind it's easy to shoot a tight group with little to no wind, but it's really hard to shoot a tight group when you have to hit the center of the bullseye repeatedly on different bullseyes, or the heart of an animal at a great distance. This is why we typically do not shoot animals beyond 300 yards, because it falls back on ethics and POI shifts due to endless variables.
greenejc
Well-Known Member
Yes, its normal. You're higher, air density is lower and temperature is lower. Whenever you change altitudes by more than 1000 feet, there will be a change in impact. I live here in Colorado at 7000 feet, and hunt at between 8 and 9000 feet. I verify my rifles at the hunting site to insure they're still on target because there's a big difference in air density between 7000 and 8000 feet. Also, the higher you get, the faster the air density/air pressure drops. You can verify this by running your bullet, velocity, G1 BC and bullet weight through a ballistic trajectory calculator and simply changing the elevation figures. The drift left may have been wind, which was always a factor in Kansas when I was stationed there at Fort Riley. There always seemed to be an east or west wind blowing at 8 to 10 mph, and that may be what you had. You'd get about a 1/2 in drift from that. Just for grins, I ran your bullet and velocity at sea level, and at 2500 feet. There's about a + 25 fps change in average velocity, which could be why you saw a shift in impact. The bullets got there faster so they impacted low (slightly).
George Koch
Active Member
There was at least a 40 degree temperature change between New York, almost 2100 feet in altitude, plus you were flying on a plane. I assume you have a quality scope, did you check it before you Shot to make sure it had not loosened up, did you use the same batch of Bullets powder and seating depths.
Same type of bench or Prop in Kansas, unless you were in a hurricane at 100 yards shooting 175 AB I doubt it was the wind.
And one of the biggest wonders not knocking the City as I live out in the rural country with big open spaces, are you use to shooting in the outdoors.
Rule of thumb is, and we always make the hunter test fire the rifle that we guide the day before the hunt or the afternoon that they arrive in our presence before a hunt regardless if they travel 10 miles or a 1000 miles, I've seen a many of one that didn't hold from their home to our camp and the most miserable hunt is missing the trophy over a gun that has lost zero for whatever the reason.
We've had a few show up with a zeroed rifle only to find out it was only bore sighted and that's what caused us to make sure we watched them throw rounds downrange.
Yes 100%! The factors that affect trajectory specifically are altitude ( better known as barometric pressure, temperature and grains of moisture! I live in Colorado and zero at 5,298 ft at 80 degrees, I hunt above 10,000 feet at 20 to -15 degrees. Those differences can be as much as 9" to 14" differences. That is why most hunters in this types of conditions are now dialing adjustment based of of ballistics apps. A couple of good application are Applied Ballistics, StrelokPro Pro (my go to), BallisticsARC, BullitDrop+ and Shooter. I have all and have done extensive testing and have found StrelokPro to be the most accurate when tethered to a Kerstal D3 Drop weather meter.
greenejc
Well-Known Member
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.
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If your gun didn't get banged around then that's the reason for the change in POI, adjust to your area of hunting and get it zeroed to where you want it and it's repeatable and your good.
Lighting conditions will change POI based on perceived sight picture.
I will be high going from an indoor range to outside and the same amount low going the other way.
Heavy mirage will make your impacts high based on the the perception of the target image floating.
If you're low and also have windage then it was probably positional on your part or some trauma during transport.
I will be high going from an indoor range to outside and the same amount low going the other way.
Heavy mirage will make your impacts high based on the the perception of the target image floating.
If you're low and also have windage then it was probably positional on your part or some trauma during transport.
Hookdown
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All these responses, and no one has mentioned the drizzle as a cause? Who knows how many water droplets that bullet goes through before impact? Water droplets are thick and heavy compared to even thick, high humidity air. Plus the momentum of each droplet would impart a downward force vector on the bullet. Your impact was low, even though your altitude was higher. I'm not saying that the drizzle is the complete answer to your question, as I suspect shooting/ rest differences too. But I think it may be a contributing factor.
chickm1
Well-Known Member
An old Marine sniper proved to me, how the changing sun height, caused the mirage to shift the target, as the sun moved in the morning.
greenejc
Well-Known Member
What range are you shooting on? Are you at Fort Riley, or Kansas City? I spent 14 years at Fort Riley and worked for Range Control as an NCO for 5 of those years. I'm very familiar with all the small arms ranges and both POF ranges under all weather conditions, because I ran most of them and worked on all of them. I also really enjoyed the Whitetail hunting in area 8 both on and off of Fort Riley. I had numerous farmers who would let me hunt their land.
SilverbulletMAG
Well-Known Member
Post #3?
