As the name implies, a custom drag function is specific to
a particular bullet such as a 0.284 caliber 130 grain Sierra
MatchKing, or a 0.308 caliber 154 grain Lapua Scenar. Custom
drag functions are produced by analyzing shooting data taken
by firing the specific bullet under known conditions. The data
is often produced by Doppler radar, but other methods can be
used as is evidenced by the fact that most of the standard drag
functions in use today were produced before Doppler radar was
Custom drag functions have become popular with long range shooters
and Lapua (Nammo Lapua Oy, Finland) understands this and has
made available a download of over 50 of their custom drag functions
from their Resources
web page. No need to download and learn QuickTARGET,
however, as starting with version 6.6.0 Ballistic Explorer supports
custom drag functions utilizing the same format Lapua uses.
You can now apply Ballistic Explorer's many advanced features
to custom drag functions.
In January 2017 Lapua gave us permission to distribute their
custom drag function files with Ballistic Explorer. If you installed
Ballistic Explorer after January 2017 most likely you already
have the Lapua files installed. Just follow the help
instructions to activate custom drag functions.
We're leaving the download
instructions below as a reference.
The image below is from the Lapua web page and shows the download
link for the drag files.
Resource Web Page
Click the link indicated by the yellow arrow we added to the
image above and download the zip file to your computer. Copy
the drag files out of the zip file to the default folder shown
in the Custom Drag Functions dialog as shown in the image below.
The Custom Drag Functions dialog is opened from the Options
menu in Ballistic Explorer.
You can also copy drag files to another folder and then specify
that location in the Custom Drag Functions dialog. There's lots
more information in the Custom
Drag Functions help page.
The following information is specific to the "qtu_lapua_data_drag-november_2015.zip"
download. As new downloads are posted we'll update this information
To view the drag functions in Ballistic Explorer open the Select
Drag Table dialog by clicking on the button just to the
left of the BC parameter in any of the Trace windows.
The standard drag functions are listed first followed by any
custom drag functions. Just after copying the files from the
Lapua download the first two lines for custom drag functions
list "Bad Tbl" as shown in the image below.
As the drag function files are loaded, Ballistic Explore tests
them to insure they are valid, and if not, lists them in red
as Bad Tbl. If you select a line and click the Info button
you'll get a message about that drag function file including
it's file name and location. In the case of a bad table the
message states what's wrong with the file as shown in the image
You can ignore bad tables, delete the
file if it's not a bullet you use, or fix the file. If you decide
to fix the file you can use Windows Notepad (in the Accessories
group under the Windows Start button). Open Notepad and click
the Open... option under File at the top of the
window. Next change the file type from Text Documents to All
files as shown in the image below.
With All Files set you can navigate to the location where you
saved the Lapua files and open "308-lapua n558 11,0g (170gr)
naturalis_radar.drg" as shown in the next image.
Change the period to a comma as shown in the above image and
save the file. Close (if it's open) the Select Drag Table dialog
and open it again and you should see only one "Bad Tbl"
listed as the file just fixed is now listed as N558 further
down the list.
Select the remaining Bad Tbl and click the Info button
to get the following message.
To fix this table open the "7mm-lapua gb554
11,66g (180gr) scena-l_radar.drg file in Notepad as shown below.
The velocity (Mach number) goes from 0.400 on
line 4 to 0.3820 on line 5, while the drag coefficient stays
the same. The velocity should increase from line to line and
this line is erroneous, so delete the entire line and save the
file. Reopen the Select Drag Table dialog and you shouldn't
see any "Bad Tbl" listings as the file just fixed
is listed as GB554 further down the list.
There are no more "Bad Tbl" listings but the drag
table currently named "L339299" should be "B557",
but it's not because of the error shown in the image below.
Insert the missing space and save the file. Reopen
the Select Drag Table dialog and you'll see this table is listed
The drag function table names must be unique, so as the custom
drag function files are being loaded Ballistic Explorer will
add a suffix to the end of a duplicate table name to make it
unique. Open the Select Drag Table dialog and scrolling down
the list you'll see "API526" and it's duplicate "API526-1"
as shown in the next image.
Looking at the corresponding files in File Explorer (Windows®
Explorer prior to Win 8) we see that the file associated with
"API526-1" has an older date as shown in the image
The newer file has a space just before "_radar",
which makes it a different file. Likely that was a mistake as
without the space the old file would have been overwritten.
Just delete the old file to remove the duplicate name.
Scrolling down further you'll find another set of duplicate
drag function table names as shown in the next image.
It turns out the drag functions are identical, but one has
a misleading file name. Given the bullet diameter is 0.264 inches
the file name "243-lapua gb504 6,5g (100gr) scenar_radar.drg"
is incorrect, so delete that file.
You now have 54 custom drag functions that you can use in Ballistic
Explorer just as you would use G1 or G7, except they are for
a specific bullet and have a default BC of 1.
Below is a list of Lapua product codes, which are used for
the drag function table names (when given) and the correspoinding
Lapua product numbers. The list is not complete as the webpage
seems out of date.