[Suhiir]

Assuming you have the CD version (with full Op Fire filtering):

1) Hit "Y" to bring up the Op Fire menu
2) Hit the "Filter" button
3) Set "Steel Armor Front MAX" to 255
4) Set "Steel Armor Front MIN" to 1 (I often set to 10 so ATGMs aren't wasted on APCs)
5) Set "Armor Engagement Range" to whatever the range (in hexes) of your ATGM is
6) Set other setting as desired
7) Hit one of the three Blue disc buttons to save these settings
8) Use the "N" and "P" keys to cycle to your ATGM
9) Hit the matching Red disc button (if you saved in Blue #1 hit Red #1)
10) Hit the "Apply" button
11) Hit the "Exit" button

Walla !