BC Truing. Waterline to 1,000, high at 1 mile.

[rybe390]

Hey folks!

So, I'm looking at a semi confusing scenario right now. I just took my first precision class and we spent a good bit of time on data gathering and ensuring we had quality data.

We zeroed, went over a Chrono, and went straight to 1,000 yards for trying. I was shooting a Hornady 140 bthp, and we slid my g7 bc down to .255 from .285 to get the data to line up. We confirmed at 1,000 and in and I was waterline at all distances.

I walked out to 1350 and made solid impacts there.

The next day, we had a bit of time and I took a poke at the 1 mile target. Took 10 rounds to get on, and then hit 5 in a row. The odd thing, is that I was hitting 2 mils high of trued data.

So, looking back at all the data after class, i cannot get my data to line up for both what I observed at 1,000 and in with perfect data, and what I observed at 1 mile. I'm either perfect at 1,000 and then give too much adjustment at 1 mile, or I'm good at 1 mile and I will impact low everywhere else. I've played with the BC, and then played with MV, and can't get them to line up.

I've used AB mobile to confirm data to 1,000, and used 4dof and changed the drop scale to 1.06 to line up with the 1,000 yard data. Both calculators give too much dope for 1 mile.

Has anyone see this before?

 

[CoryT]

Well, first thing first. stop tweaking the software and plot the curve. If you are good from 1350 and in, the BC and MV do not need to be adjusted. Does the 1760 elevation line up if you look at the curve? There are numerous possible problems. Bad range, bad MET data, vertical wind, ammo changes, scope error, user error, position/light/parallax.....

What we know is there is some kind of problem with one particular range session on one particular target. We can also say with a reasonable degree of confidence that the settings for MV and BC from the initial 1K session are accurate, although that is a rather large change in BC.

Was the scope calibrated? Is 1760y near the end of travel by chance?

 

[seansmd]

Did the DA change day 1 to day 2?

 

[Steel head]

This could be so many things.
Air conditions?
Wind?
Mirage?
Scope verified?

 

[st1650]

Mirage/ heat shimmer could cause this.

Have you tested your scope for the full elevation?

 

[rybe390]

Well, first thing first. stop tweaking the software and plot the curve. If you are good from 1350 and in, the BC and MV do not need to be adjusted. Does the 1760 elevation line up if you look at the curve? There are numerous possible problems. Bad range, bad MET data, vertical wind, ammo changes, scope error, user error, position/light/parallax.....

What we know is there is some kind of problem with one particular range session on one particular target. We can also say with a reasonable degree of confidence that the settings for MV and BC from the initial 1K session are accurate, although that is a rather large change in BC.

Was the scope calibrated? Is 1760y near the end of travel by chance?

Agreed, the change in BC is large. But, it didn't worry me too bad as it is a 1.06 change in the Hornady app for the same output, which is within their scope of adjustment. But then neither would give me the impact I saw at a mile.

I know you can use Dsf in the kestrel, and it was .93 at Mach .854. does that seem reasonable?

I was maxed out on the scope. I had like 18 Mils dialed and held the last 7. The scope went to 19.5 but I went back down to 18.

I have tall target tested this scope but only to about 10 mils, and everything did look good.

 

[rybe390]

Did the DA change day 1 to day 2?

Only by a few hundred feet, solution was pulled from a kestrel that was live.

 

[rybe390]

This could be so many things.
Air conditions?
Wind?
Mirage?
Scope verified?

Air was similar but a bit more sense.
Wind was fishtail shifting, 3-7 mph.
Mirage was non existent, first thing in the morning.
Scope had been tested to 10 mils and was good. The scope was maxed out though and I was holding on top of my dialed elevation.

 

[338dude]

Hey folks!

So, I'm looking at a semi confusing scenario right now. I just took my first precision class and we spent a good bit of time on data gathering and ensuring we had quality data.

We zeroed, went over a Chrono, and went straight to 1,000 yards for trying. I was shooting a Hornady 140 bthp, and we slid my g7 bc down to .255 from .285 to get the data to line up. We confirmed at 1,000 and in and I was waterline at all distances.

I walked out to 1350 and made solid impacts there.

The next day, we had a bit of time and I took a poke at the 1 mile target. Took 10 rounds to get on, and then hit 5 in a row. The odd thing, is that I was hitting 2 mils high of trued data.

So, looking back at all the data after class, i cannot get my data to line up for both what I observed at 1,000 and in with perfect data, and what I observed at 1 mile. I'm either perfect at 1,000 and then give too much adjustment at 1 mile, or I'm good at 1 mile and I will impact low everywhere else. I've played with the BC, and then played with MV, and can't get them to line up.

I've used AB mobile to confirm data to 1,000, and used 4dof and changed the drop scale to 1.06 to line up with the 1,000 yard data. Both calculators give too much dope for 1 mile.

Has anyone see this before?

There are two things that would I would suspect the first one being your BC from what I read about the changes you made it sound like you were running 1BC which is only an average and may not work at all distances you need to try running a custom curve or using an app that will allow you to build your own I like the Shooter app it lets me do that, Also not all scope track true the Shooter app also has a scope correction factor that allows you to true you’re dope when dialing up the scope if it does not measure correctly doing a tall target test can give you an idea of the accuracy of your scope tracking as well try running these multiple BC’s from Bryan Litz his book ballistic performance of rifle bullets volume three see if that helps first then check your scopes tracking

 

[CoryT]

At this point I'd need to see your complete set of inputs for both days.

 

[rybe390]

At this point I'd need to see your complete set of inputs for both days.

For those who care!

I was able to recreate the same exact data by adjusting MV. I have a feeling my ammo was baking in the sun before chrono, and that's why I had to true so badly. I've shown every scenario below, from recorded impacts, to trued data, and my attempt at rebuilding the data without using very large BC changes or DSF curves.

Original MV and BC: 2674 fps, G7 .255 adjusted. DSF at Mach .854 of .93
Adjusted MV that matches above data set: 2630 fps, G7 .285 non adjsuted, no DSF truing

Environmentals:
25.17 inhg
80.5 degrees F
40N
DOF: 135 degrees
7300 Density Altitude

Recorded Data, confirmed impacts
283 - 1.0
365 - 1.7
415 - 2.1
543 - 3.3
625 - 4.2
1,000 - 9.2
1348 - 15.0
1743 - 24.7

Data at 2674 g7 of .255, no DSF applied
283 - 1.0
365 - 1.7
415 - 2.1
543 - 3.3
625 - 4.2
1,000 - 9.2
1348 - 15.9
1743 - 26.9

Data at 2674 g7 of .255 DSF applied

283 - 1.0
365 - 1.7
415 - 2.1
543 - 3.3
625 - 4.2
1,000 - 9.1
1348 - 14.9
1743 - 24.7

Data at 2630 FPS, g7 .285. Theory that ammo was very hot when chronoed.

283 - 1.0
365 - 1.7
415 - 2.1
543 - 3.4
625 - 4.2
1,000 - 8.9
1348 - 15.0
1743 - 24.8

 

[Steel head]

If you have DSF use it.
My 7 mm shoot under predicted a bit at subsonic and DSF got it in line to my actuals.
I don’t mess with it on one day of data unless I’m 100% confident in the actual conditions and distances.
It has to be repeatable before I change it.

 

[Long Range 338]

Well, first thing first. stop tweaking the software and plot the curve. If you are good from 1350 and in, the BC and MV do not need to be adjusted. Does the 1760 elevation line up if you look at the curve? There are numerous possible problems. Bad range, bad MET data, vertical wind, ammo changes, scope error, user error, position/light/parallax.....

What we know is there is some kind of problem with one particular range session on one particular target. We can also say with a reasonable degree of confidence that the settings for MV and BC from the initial 1K session are accurate, although that is a rather large change in BC.

Was the scope calibrated? Is 1760y near the end of travel by chance?

Some great advice here, especially in the first paragraph. Many have had similar experiences and often the best fix is to create a separate profile for the rifle/load combo at ELR. Often the biggest problem (and this is from personal experience) is you don't know what you don't know.

 

[Steel head]

Some great advice here, especially in the first paragraph. Many have had similar experiences and often the best fix is to create a separate profile for the rifle/load combo at ELR. Often the biggest problem (and this is from personal experience) is you don't know what you don't know.

One important tid bit is unless your 100% you have your environment dialed (which is rare in my experience) it usually take 2-3 trips to get a rock solid true.

 

[rybe390]

I used my adjusted and trued data in a match this weekend, and data was dead on to 1200 yards.

I am going to chalk the original variance up to needing to tweak the BC super far due to the MV shift of hot ammo over the Chrono. I chronoed another batch of ammo, and it had a lower MV. That MV coincidentally lined up with almost the exact observed data, without a bc tweak. That ammo then shot in the match had hits to 1200.

Lesson learned, don't Chrono sun baked ammo!

Side note. How do folks keep ammo cool at matched?

 

[Steel head]

I used my adjusted and trued data in a match this weekend, and data was dead on to 1200 yards.

I am going to chalk the original variance up to needing to tweak the BC super far due to the MV shift of hot ammo over the Chrono. I chronoed another batch of ammo, and it had a lower MV. That MV coincidentally lined up with almost the exact observed data, without a bc tweak. That ammo then shot in the match had hits to 1200.

Lesson learned, don't Chrono sun baked ammo!

Side note. How do folks keep ammo cool at matched?

Develop a load in a good speed node with a powder that doesn’t freak out in the heat and have data for cold, hot and normal temps if needed.

 

[Sig Marine]

If you’re using a powder that is temperature stable and have chosen a node that has a wide window of accuracy, find out the velocity change per degree of increase in temp for your powder relative to your zeroing temp and use that in your ballistic program. Your software will automatically adjust your elevations, if you select to do so, when you put in current temp; based on the change in temp and the adjusted MV. The program will assume your powder is at ambient temperature so just keeping it in the shade or covered, i.e. do not let it sit in the sun and bake, is usually good enough.