Not getting a good trace, and can't figure out why.

[The King]

So - I'm not getting a good trace out of my ballistics programs right now with a certain setup, and I don't know why.

Rundown of the situation:

75GR hornady steel match.
2559 FPS as recorded over 10 rounds with a

2.7 Height above bore
1/10mrad clicks
1/8 twist
Zero is at 94m/100yds, and is .05mrad left

24.47 in/HG baro, measured in a sig 2400, an iPhone barometer, and local weather station.
6562 F elevation
18% humidity

5 degree up angle inclination wise

Programs used:

TRASOL
Ballistic AE
Applied Ballistics
Hornady 4DOF



800m shot - actual impact on target at 9.5 mils - I show 8.67
460m shot - actual impact on target at 3.7 mils - predicted 3.0

These are giant steel targets painted tan with a big aiming dot on them to aim at - impact spotting is exact and measured with an MSR reticle scope.

Groups were shot on each target to see if it was a dispersion issue, the groups were fine but their location on target was not.

I cannot seem to get the two numbers to line up in any situation really, and multiple solvers say the same thing within a few hundredths of a mil.

What am I fn'g up?

 

[TheGerman]

Internal scope tracking variance/error

Have you done a tracking test?

 

[The King]

Not on this exact optic- that's the next range trip. :-(

Its almost the only thing I can think of. It even does this with the AB CDM.

 

[TheGerman]

If there's no shift between rounds, or one group is 5.4MIL and the next is 5.9MIL or whatever, and all your ballistics programs say the exact same thing it basically has to be either your velocity is off, the BC of the bullet you are using is inflated/wrong or your optic's tracking is off.

 

[The King]

Velocity is from the lab radar - that doesn't mean it can't be off but its less likely.

Even tweaking the velocity doesn't help - when you think about it its saying "correct more at 460m" and "correct less at 800m"

I haven't managed to figure this one out - it doesn't match any failure model (like badly measured or assumed BC) that I'm aware of.

The only thing I can think of is highly localized and strong wind coming towards me and then dissipating. Which could be happening - the backstop is a mountain.

 

[TheGerman]

Velocity is from the lab radar - that doesn't mean it can't be off but its less likely.

Even tweaking the velocity doesn't help - when you think about it its saying "correct more at 460m" and "correct less at 800m"

I haven't managed to figure this one out - it doesn't match any failure model (like badly measured or assumed BC) that I'm aware of.

The only thing I can think of is highly localized and strong wind coming towards me and then dissipating. Which could be happening - the backstop is a mountain.

This part has me thinking tracking.

 

[delfuego]

Give up on those 75gr Hornady's. I tried the "Steel Match" and they shot great at 100y, could hit shit past 500y. They fall apart after 500y. Try some 77smk. Do they track to 400y? That day I switched to IMI 77gr, and was back on steel.

 

[lte82]

From applied ballistics website:

Atmospheric inputs have historically been the least understood and caused the most trouble for shooters running ballistics programs, in particular the pressure inputs. The following discussion elaborates on the correct way to manage these variables. Basically there are two options for describing pressure to a ballistics program; 1) Enter the barometric (aka corrected) pressure and altitude, or 2) Enter the Station pressure where you are. Some definitions are in order regarding barometric and station pressure.
Barometric pressure is also known as sea level corrected pressure, and is what the weather station and airports report because it’s useful for pilots and making weather assessments. Barometric pressure is not the actual air pressure where you are, rather it’s a number that’s corrected to sea level. In order to determine the actual air pressure where you are (which is what the ballistics program cares about), you have to account for the effects of altitude. However if you have a handheld weather meter like a Kestrel, you can measure Station Pressure directly which is the actual air pressure where you are. This is the preferred method of inputting pressure data because it’s one less input and relies on only one measurement instead of two.
A common error is to mistake station pressure for barometric or vice versa. The consequence of this error is that the wrong air density gets applied which degrades the accuracy of trajectory predictions. This error is increasingly more severe the higher up you are above sea level.
Refer to the image on the right for proper set-up of the atmospheric pressure inputs. Note the reference altitude is set to 0 ft in the Kestrel which indicates it’s displaying uncorrected station pressure, and the Pressure is Absolute box is checked in the program indicating it’s using station pressure.
To further clarify the output from the Kestrel, here is an excerpt from the Kestrel user’s manual: “Some final notes – If you wish to know the actual or station pressure for your location (such as for engine tuning), simply set the reference altitude on the BARO screen to “0”. In this case, the Kestrel Meter will not make any adjustment and will display the measured value. (Engine tuning and ballistics software sometimes refer to atmospheric or station pressure as “absolute pressure.” These applications are concerned with the actual air density, as opposed to pressure gradients relating to weather, so barometric pressure is less useful.”
Altitude – This is your altitude above sea level and is only used if you’re working with barometric pressure.
Barometric Pressure – If using altitude, input Barometric pressure. If not using altitude, enter measured station pressure at your location (see above discussion under Atmosphere).
Pressure is Absolute – [Checkbox] If checked, it means you’re entering station pressure as measured by a Kestrel (or similar device). If unchecked, then you’re working with altitude and barometric pressure.
Temperature – This is the air temperature thru which the bullet flies.
Humidity – Relative Humidity in %. This is not a highly important variable, if unknown enter 50%.
Wind Speed – Enter the speed of the wind.
Wind Angle – The angle/direction of a wind is named for where the wind comes from, and you can enter this in either degrees or clock direction. For example, a wind blowing from right to left is a 3 O’clock, or 90 degree wind. A tail wind is a 6 O’clock or 180 degree wind, etc.

 

[The King]

Thank you for the pressure heads up - I did have a good handle on that one.

I used the inbuilt mechanisms (measurement of station pressure) in the Sig 2400, as well as internet connected data gathering in TRASOL and the others.

All of them agreed, and changing from station to corrected presssure and entering the new numbers (29.7 in/Hg if I remember right) didn't make a difference.

 

[lte82]

If I use 24.4 inhg and 45 degrees / 18% rh with a 75 eld @ 2560 my app shows 9.4 mil @ 800m.

 

[lte82]

Also try truing the bc to .205.

 

[The King]

Its a 75BTHP. Unless hornady changed what they loaded into the 75 steel match. And I can get 9.5 at 800m all day - but I can't seem to get it to do that AND 3.7 at 460m.

 

[The King]

I'll be back to this problem with a DETAILED run through of the situation both externally and in the calculators.