You might be thinking of Douglas Cole's ballistics spreadsheet, which was based on an old Pyramid II article he did.
The most current version of the spreadsheet is available here:
https://docs.google.com/file/d/0B9-z...18LZ-JlIwnG0Tw
Note that there are numerous limitations to this spreadsheet, but it is still extremely useful since it provides a good formula for converting Kinetic Energy imparted by high speed projectiles into GURPS dice of damage.
Another good GURPS ballistics program is here. It's major limitation is that stats are heavily modified to fit the author's house rules.
http://panoptesv.com/RPGs/Equipment/stuff.html
Between these sources and FFS and possibly the BTRC G3 supplement you ought to be able to stat out just about any GURPS Traveller weapon.