Adjusting ballistics app data help

[TBuckus]

Hopefully someone can walk me thru this.
First time shooting with a turret over 300 yds. I plugged in all my info into my ballistics app and shot 3 shots at 500 yds. Shot a 2-3/4" group about 2.5" high and 3" left. App said to adjust 7moa
I didn't make any adjustments for wind on this group so I can contribute the shift left.
Using the same data I went out to 875, 17moa and I was shooting high, missing the 12" gong.
So, I'm trying to figure out where to start. I double checked my data with other calculators and came up with the same data.
Do I tweak the BC or other data?

 

[Fin-addictions]

First, where is your muzzle velocity coming from? Have you chronographed the load? Have you chronographed enough shots to have a good average?

What BC are you using? If it's a G1, and you've chronographed your load, I'd look for a Litz verified G7. G1 BC's degrade as the velocity drops, so once your bullet gets below 2000 FPS, there will be divergence in your drag curve.

Are you using good atmospheric conditions? Primarily the actual elevation you're at, along with the station pressure. It can't be corrected barometric pressure for your altitude, but the actual station pressure for the location you're at. Without this data being good, you'll also begin to see divergence from the predicted bullet flight.

 

[dfanonymous]

Hopefully someone can walk me thru this.
First time shooting with a turret over 300 yds. I plugged in all my info into my ballistics app and shot 3 shots at 500 yds. Shot a 2-3/4" group about 2.5" high and 3" left. App said to adjust 7moa
I didn't make any adjustments for wind on this group so I can contribute the shift left.
Using the same data I went out to 875, 17moa and I was shooting high, missing the 12" gong.
So, I'm trying to figure out where to start. I double checked my data with other calculators and came up with the same data.
Do I tweak the BC or other data?

True mv first, then bc. Assuming it's nothing else.

 

[TBuckus]

First, where is your muzzle velocity coming from? Have you chronographed the load? Have you chronographed enough shots to have a good average?

What BC are you using? If it's a G1, and you've chronographed your load, I'd look for a Litz verified G7. G1 BC's degrade as the velocity drops, so once your bullet gets below 2000 FPS, there will be divergence in your drag curve.

Are you using good atmospheric conditions? Primarily the actual elevation you're at, along with the station pressure. It can't be corrected barometric pressure for your altitude, but the actual station pressure for the location you're at. Without this data being good, you'll also begin to see divergence from the predicted bullet flight.

I chrono'd my loads and have consistent velocities.
I'm using the G1 BC. I'll look up the G7
Now you got me on station pressure. That is something that I'm not familiar with. How is that different that barometric pressure?

 

[dfanonymous]

I chrono'd my loads and have consistent velocities.
I'm using the G1 BC. I'll look up the G7
Now you got me on station pressure. That is something that I'm not familiar with. How is that different that barometric pressure?

station pressure is the pressure where you are vs barometric pressure is pressure corrected from sea level.

Density altitude plus temp is easier to use and is relevant to air density which is really all that matters with external ballistics.

 

[Fin-addictions]

I will add and agree that density altitude is easy to work with if you have a way to easily get the density altitude number. I have a Kestrel and it gives a density altitude. I've also got Magpul data books, and they have a density altitude chart in the front of your log. I've compared it to the density number from my Kestrel and it's always been spot on. The Magpul chart is just looking at the temp and the altitude, and giving you a density number. Pretty easy way to get it.

 

[ShtrRdy]

Lots of good advice here. One other thing to check is your scope tracking. This can be done with a tall target test. Your scope may not be moving the reticle the exact amount it says it is.