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 invented.

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.

Lapua 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 match.

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 below.

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 as B557.

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 below.

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.