Come ups in Hornady 4DOF App?

[rmiked]

I’m looking at a trajectory in the Hornady App for a 6.5 CM. The values for come ups in MOA, don’t zero out the bullet drop indicated for the trajectory. Can someone explain what “come ups” are and how they are defined? I would think it is exactly the MOA needed to be held (or turret dialed) to move the bullet to your line of sight. In other words, once you are past the zero distance, it would be how many MOA are needed to be held over (or dialed) to make the bullet coincide with your line of sight. An example is at 300 yards (with 200 zero) the trajectory is -7.64”. The associated “come up” is +2.12 MOA. At 400 yards the trajectory is -22.15”. The associated come up is 4.98 MOA. They both are about 0.3 MOA too low.

 

[Longshot231]

Perhaps the best way to learn the app is to watch some videos on it. This is a good one to start with.

 

[rmiked]

I watched the video which is very informative to set up a rifle. He didn’t address come ups. However, while reading about aerodynamic jump, which the 4 DOF app accounts for, it is the tendency for a left wind to push a right hand twist bullet down, and a right wind to push the bullet up, when the wind is 90 degrees to the line of fire. The trajectory listed in my output file is the gravity only component. I had input a wind of 10mph from the right. That wind will make the bullet rise. The approx 0.3 MOA difference (low) is accounting for the wind making the bullet rise. I reset my wind to 0 mph and the come up exactly matched the gravity drop. I made a wrong assumption that the wind affect on the trajectory was already accounted for in the output. It is not. The come up gives you the right solution including aerodynamic jump due to wind. I think this aerodynamic jupm aspect is more complex than many use. I typically account for gravity induced drop and then make a wind hold separately as if the wind has no affect on vertical trajectory. I typically don’t hunt far enough for the wind impact on drop to be an issue. The Hornady 4 DOF app seems to be very sophisticated. As far as I know, the JBM program doesn’t include aerodynamic jump. The 4 DOF output has a “no wind trajectory” column showing the gravity only solution in MOA, which differs from the “total come up” solution. Pretty neat.

 

[Rob01]

Why don’t you just use the app and not have a file? Put in your actual wind speed and direction and also other environmental data. That will give you the correct info.

Also forget inches. No need for them even if they put them in the chart.

 

[rmiked]

I agree just inputting correct parameters into the App will work. I was just trying to figure out how the App works to gain confidence in it. When I saw the “come up” didn’t match the bullet drop due to gravity, I was suspicious I was doing something wrong. I am familiar with lateral offset due to Coriolis effect and spin drift. But I have never seen a ballistic App account for Aerodynamic Jump but I mainly use JBM. It is vertical offset due to 90 degree wind passing over a spinning bullet. I believe this is just another version of spin drift in a different direction. Very impressive.

 

[Rob01]

Well sounds like you know how it works so why question it?

 

[rmiked]

I don’t question it now. The results looked odd to me at first. I had never heard of aerodynamic jump. I was accustomed to “come-ups” zeroing out gravity drop only. And that’s true for no wind or no consideration of Aerodynamic Jump. The 4 DOF output table has a column showing the specific Aerodynamic Jump component. The App seems incredible. Thanks to Longshot231 for the Utube link.

 

[rmiked]

I saw Bryan Litz of Applied Ballistics on a Utube discussing Aerodynamic Jump. He said the Applied Ballistics software accounts for it. He made the video 6 years ago and at that time no other software did account for Aerodynamic Jump. Today the Hornady 4 DOF software also accounts for it. According to Hornady , 4DOF doesn’t even use the ballistics coefficients they publish, in 4 DOF. The 4 degrees of freedom are elevation, windage, range and “angle of attack” (new 4th degree). 4 DOF uses a drag (coefficient) curve itself citing that use of the ballistics coefficient is mathematically inferior to use of the drag curve because the ballistic coefficient changes as the velocity changes and the drag curve doesn’t. I imagine since most people never used it or had the tools before to use Aerodynamic Jump, the variability in shooting results was attributed to errors; wind, bad hold, less than ideal trigger release, etc. The impact of Aerodynamic Jump out to 400 yards for my 6.5 Creedmoor seems to be about 0.3 MOA either low or high depending on if wind is blowing from left or right. 0.3 MOA at 400 yards is about 1.2”. As long as the group is good most people would probably accept 1” high or low at 400 yards without thinking about it as it’s only 1 click on the turret. For me , my shooting form and trigger technique dominate my shooting results, once I have worked up a good consistent load for my rifle. It’s really neat to learn about all these hi tech tools. And I can imagine for long range work in the 700-1600 yard range this parameter being more significant.