Range Report Observed impact vs calculator(s) output: large variation

[rybe390]

Hey folks. I have an interesting case from a recent range trip. I was impacting pretty seriously low from what multiple calculators were telling me. I first noticed at 600 yards, and took out to 1,000. What started as about a .5 mil low, ended at .8 mil low from calculator to actual impact. 9.2/9.3 mil actual vs 8.5 calculator. In my experience, this is a massive deviation from what has otherwise been spot on calculators across multiple guns.

This is a new barrel and new scope, so I'm wondering what factors could have contributed. Atmospherics were perfect so it isn't that. Rifle setup in the calcs were also good, as was input data. I've made a possible causes list below, can anyone lend a potential hand as to what can cause a large variation here. Typically I'm .1 or .2 off at most, not close to a full mil.

Shooting 147 eldm at 2620 fps. Using 4dof and ab mobile.

Muzzle velocity drop- would have dropped 80 fps from Chrono data. 80 rounds through tube, it was pretty dirty when I cleaned. Possible? New barrel was chronoed in first 20 rounds. End of range trip had 80 rounds.

Zero shift- confirmed not this at range

lower BC/form facctor - probably not this, way too drastic over 10%

Hot barrel stringing - likely not, hot barrel groups well at indoor range.

Scope tracking error - need to test this. Would be huge error, over 8%

Unstable bullet - how do you test this? Shooting 147 eldm at 2620 fps in a 1:8 barrel.

 

[Steel head]

what were your actual drops?
Check your speed
Check your zero
Check your scope tracking.
Triple check your inputs

Stability should be fine.

 

[morganlamprecht]

without knowing all of your actual inputs...in 4dof i get 9.1mils using mine for a 147 going 2620fps

barrels typically speed up when dirty or fouling in...cant say ive ever seen one slow down until throat wear comes into play 100s of rounds later

do you have a shooting angle entered by accident?

 

[Jimgal62]

Hey folks. I have an interesting case from a recent range trip. I was impacting pretty seriously low from what multiple calculators were telling me. I first noticed at 600 yards, and took out to 1,000. What started as about a .5 mil low, ended at .8 mil low from calculator to actual impact. 9.2/9.3 mil actual vs 8.5 calculator. In my experience, this is a massive deviation from what has otherwise been spot on calculators across multiple guns.

This is a new barrel and new scope, so I'm wondering what factors could have contributed. Atmospherics were perfect so it isn't that. Rifle setup in the calcs were also good, as was input data. I've made a possible causes list below, can anyone lend a potential hand as to what can cause a large variation here. Typically I'm .1 or .2 off at most, not close to a full mil.

Shooting 147 eldm at 2620 fps. Using 4dof and ab mobile.

Muzzle velocity drop- would have dropped 80 fps from Chrono data. 80 rounds through tube, it was pretty dirty when I cleaned. Possible? New barrel was chronoed in first 20 rounds. End of range trip had 80 rounds.

Zero shift- confirmed not this at range

lower BC/form facctor - probably not this, way too drastic over 10%

Hot barrel stringing - likely not, hot barrel groups well at indoor range.

Scope tracking error - need to test this. Would be huge error, over 8%

Unstable bullet - how do you test this? Shooting 147 eldm at 2620 fps in a 1:8 barrel.

Did you do a tall target test on the scope?

 

[PinesAndProjectiles]

Have you trued your data?

 

[Ledzep]

.5-.8 Mils is beyond the realm of what you should be trying to "true"/curve fit.

 

[Steel head]

.5-.8 Mils is beyond the realm of what you should be trying to "true"/curve fit.

I concur
Something is off.

 

[rybe390]

I concur
Something is off.

Agreed here and that's what I'm really troubled with.

I have yet to do a tracking test/tall target test to confirm error. But, it would be an 8% scope tracking error which seems huge.

Actual drop was 9.2 mils, calc was 8.5 mils. DA of 8,000 feet.

I need to re confirm MV, as I only chronoed before I went out. But it's a magnetospeed so I trust it. It would have been 80fps slower to get the impacts I was getting.

With confirmed zero, MV, inputs on calculators, that really only leaves scope tracking as a variable, correct?

 

[morganlamprecht]

or a shift from lighting/environmental effects...but .8 at 1k is quite a bit

 

[Steel head]

How did you get the DA?

It could be a combination of a few issues.

 

[rybe390]

How did you get the DA?

It could be a combination of a few issues.

Pulled from a Brunton ADC. Confirmed with local weather station as well as a actual BARO reading.

Starting to lean towards: MV and scope tracking.

MV needs to be confirmed after the fact
Scope tracking needs to be determined

All other factors are far too large to be of this impact. It would be like cutting the BC in half, or being off by 5k on DA.

 

[Steel head]

Pulled from a Brunton ADC. Confirmed with local weather station as well as a actual BARO reading.

Starting to lean towards: MV and scope tracking.

MV needs to be confirmed after the fact
Scope tracking needs to be determined

All other factors are far too large to be of this impact. It would be like cutting the BC in half, or being off by 5k on DA.

What BC did you use?

 

[rybe390]

What BC did you use?

I used the AB custom drag curve, and the provided Hornady Doppler in 4dof. Both of the calculators gave the same output.

 

[Ledzep]

Scope height over bore and scope tracking would be the two main culprits I'd check first.

 

[rybe390]

For anyone who cares, I went out again this weekend, and had a hugely successful day.

I confirmed zero with my 140 eld, and ran a tall target test up to 8 mils. The scope was tracking perfectly, so, I was able to put that to rest.

140 over the Chrono was also good with prior data.

We shot to the furthest the range goes, out to 600 yards. Hitting MOA targets at 5 and 600 yards on first round impacts, I'd say everything is resolved and the 147s impacting low was a fluke.

I just am going to stick with the 140 class bullets, no reason to try and fuss with something to try and figure out what was wrong, when you have something else that works great.

So, tracking error ruled out, Chrono data seemed good. Live fire with different ammo and the results spoke for themselves. Happy as can be! Hitting a 5" hostage popper at 600 yards a few times in a row puts everything at ease about your system.

 

[1moaoff]

Nice