You just need to get weapons.txt extracted from Sheets.vol, and modify the weapon values. Mainly the penetration damage, and concussion damage values. Armor can block some concussion, but not penetration.
To get that file out of Sheets.vol, you can find a utility to extract it, like the map editor should probably do, or search for the file on the forums. I'm pretty sure that file is posted somewhere in the file forum in a thread relating to the VOL files. Originally the files in the VOLs were compressed, but a VOL with uncompressed files was posted there. However the VOL file utility that existed at the time seems to have had a slight bug and couldn't extract one of the files, so it was posted seperately later in the thread as a simple .txt file. If I remember correctly, it was weapons.txt. (Note, the old utility isn't used anymore since it could never handle the compression/decompression, and was a little buggy for the extraction, and didn't fully support repacking arbitrary files into VOLs).
So yeah, copy that file to the Outpost2 folder and edit it. Any settings in there will override what's in the VOL files. It looks in the Outpost2 folder before it checks the VOLs.