What would cause this? Bullet drop with elevation change

[Stevenstl86]

I'm curious to ask what the mv was for the load worked up? And was the 100yrd target verified @ 100yrds at both locations?

 

[sodakota]

Using powder that's temperature sensitive? That would be my guess.

 

[D.Camilleri]

Question #1 you state you mounted a scope and sighted in the gun for your friend, did he shoot the gun afterwards and verify that he is hitting to the same point of impact? Different people can look through a scope differently changing point of impact. I have also seen an increase in velocity cause a bullet to hit lower at 100 yards.

Some things can make you scratch you head. Before my elk hunt this year I did some range time with one of my 338 rums. I checked zero at 100 and took it out to 700 and every thing was good. I shot a bull at 470 yards and I almost missed him by hitting high. I ranged him and added an additional click because he was walking away and then I shot off of my Huskemaw tripod and heard the bullet smack. When I got to the bull, I had to finish him off, because the bullet hit much higher than my aiming point taking out the very top of his back. I went back to the range when I got home and found my gun was now hitting 1.5 inches high at 100. I did a lot of bouncing around on an atv on my hunt , and when I made the adjustment to my scope, my dope was right on again out past 700 yards. There is another variable here also. I have found on several occasions that when shooting off of this tripod or even off of shooting sticks that I am hitting high. A buddy has found the same thing and contacted Best of The West and they told him to extend the front legs of the tripod out two inches more than the rear leg to allow loading of the bipod. I need to get out and try this. Also, a person needs to shoot from the same position when sighting in as when hunting. I.E. don't sight in from a bench and then shoot prone at hunting camp.

 

[7mmTikkaShooter]

This seems too basic to consider but I have to ask; are they all shooting at 100 yards? If they are assuming the yardage then one could see how it's possible. A gun zeroed at 100 yards, and shooting at a 200 yard target, will be hitting low regardless of elevation or temp. I have to believe that these 6 hunters would have checked the yardage but that would explain all 6 having the same issue that doesn't seem to have a logical scientific explanation.

 

[riffraff]

Just one other possibility... light refraction in the scopes. I have seen this numerous times at one range I shoot at... it is oriented to the southeast, and as the sun gets higher in the morning the elevation required to be on target increases by +.2 mils. Horizontal correction is .1 mils as the sun traverses from morning to afternoon (left to right for the range direction of fire). The correction is always 'toward' the sun. You can confirm this yourselves at the range by setting your rifle crosshairs on the target for a few hours and check it occasionally. Here is a video that demonstrates it (go full screen for best visibility)...

And an article... https://thearmsguide.com/5331/long-range-shooting-external-ballistics-light-effects/

A similar effect happens on flat ranges when there is a layer of air close to the ground that is different from the air a few feet above the ground... happened to me in Tennessee once in early morning where the cool ground temperature made a cool layer low to the ground, and everyone was shooting .2 mils low on our DOPE checks from 300 out to 1000 yards. https://en.wikipedia.org/wiki/Atmospheric_refraction

Don't know if this is the OPs problem, but since it happened to all of them, it might be worth considering.

 

[Hugnot]

It comes across like these 6 guys had 6 rifles that were sighted in at some home range then traveled to a distant location using different 6 vehicles. Upon arrival, at the distant location, all six rifles did not have the same point of impact(s) that were originally obtained at the home location. The distant location was of a higher elevation and of different environmental conditions.

My guess is that the range distances and sight angles (up-down) were different at the remote site. At 300 yards the bullet path being sort of like a parabola means like relatively small increments of distances will result in greater bullet drops.

My Sig Sauer range finder has problems at low temps and sometimes gives funny results, like an indicated 350 yards might actually be 400 yards and the extra 50 yards would be on the increasing downward slope of the parabola. 350 yards is a relatively short distance for air density changes to have big impact shifts.

I don't think that the internal event, like blasting a nitrocellulose propellant like H1000 by a primer explosion in a cartridge would be affected by the these external environment changes.

Good idea to check things out upon arrival at some distant & different location.

 

[Deleted member 115751]

There is an old theory of fluid dynamics that was developed in the early part of the 20th century called "Boundary Layer Theory". Basically, a thin film of fluid (air in this case) stays stuck to a surface of an object as it moves through it. However, at some point in the surface the boundary layer will separate from the surface. To calculate the drag you would have to integrate the differential equation that describes it across the exposed surface of the object where the boundary layer has separated. What I am proposing is that the the boundary layer formation is affected by the density of the fluid (aka altitude above sea level affect the density of air) which in turn leads to a higher drag which slows down the bullet allowing gravity more time to act on it in the downward vector. Obviously this is just a hypothesis that has not been tested but it assumes that a lower fluid density causes the boundary layer to separate sooner.

Boundary layer - Wikipedia

en.wikipedia.org

Flow separation - Wikipedia

en.wikipedia.org

This link from NASA is probably the simplest explanation I can find to describe the behavior of an object moving through a fluid.

This site has moved to a new URL

www.grc.nasa.gov

"To make things more confusing, the boundary layer may lift off or "separate" from the body and create an effective shape much different from the physical shape. "

 

[Jeffrey Van Zandt]

So just out of curiosity what type of range facility and benches are they using at elk camp ? Are the targets on the same plane as they were where you sighted in at not up or down angled from at the home range . Is it close to the base of a cliff , or maybe in a canyon or large draw ? We have a lot of changing air currents where I live when you are at the base of a cliff depending on the direction of the wind hitting the back side of the cliff you can have a down draft or if it's blowing toward the cliff face on your side you can have an updraft . Down in Colorado at the Bolder air port they have what are called rotators that are similar to a tornado laying on it's side but you can't see them with your eyes as they don't pick up any ground litter and before they learned what was happening there were several small planes that just all of a sudden nose dived dropping several hundred feet in seconds . Perhaps something of this nature is happening so that there is a hard downward wind between the shooter and the target . Just some thoughts I had from some of the things I've seen in the mountains that people from other places may not have seen yet .

that is why after I get my 100 yard zero locked in and then true my kestrel to the round at 1200 I shot rock, ledges with cracks ect at all different ranges and times of the year up here from spring to fall and in to winter so temp change rests for gun change altitude and temp along with different winds just to be ready for taking my winter meats when the shot comes

 

[73driver]

Generally speaking the higher in elevation the flatter the trajectory. However if you sighted in at a dryer climate but are hunting in high humidity that could offset that elevation effect with a slower bullet flying through denser air. Temperature could be an additional factor. Your MV at 80 degrees will be faster than that same powder charge at 30 degrees. Ive read some guys keep their ammunition in a protected pocket from the cold and load only when ready to shoot so the round isnt effected by heat OR COLD temperatyres.

Dry air is more dense than humid air. Of the inputs to a ballistic calculator temperature, air pressure (true station pressure not the more common cited corrected pressure ) and humidity, humidity has the least impact.

 

[DSheetz]

riffraff , like looking at a fish in the water .

 

[Chrsm]

Gravity makes me shorter as time goes on!

 

[Shane Lindsey]

Did they all zero with a bipod and then get to camp and shoot off a pack? That can change POI.

 

[TheLongRanger83702]

Gravity makes me shorter as time goes on!

Could you please stand up so we can see who's speaking? Oh... sorry...lol.

 

[epoletna]

Chrsm: I've noticed that gravity and time make me not only shorter, but make my waist bigger!