By "search" I assume you mean using Windows search? That's a bit more workable for XP, but for some reason Windows 7 isn't very good at finding these entries, even though I indexed the game directory...
The "class system v5 beta" WEAPONINFO_ALT fixed my issue; when renamed to WEAPONINFO, it no longer causes an editor CTD.
Should I be extracting the rest of those files (ADDONINFO, AMMOINFO, GRENADEINFO, and MINEINFO) for stability's sake though, considering that I'm running regular non-class BSM?
I vaguely remember seeing the XM-18's model and its ammo in regular/original BSM WEAPONINFO, so I'm actually not sure
why you had a WEAPONINFO_ALT, as trying to glance through them didn't make me see any additional weapons, just different values for certain weapons' stats (i.e. Ready time, or Wear/Durability), much less what the "class system v5 beta" WEAPONINFO_ALT changed compared to them, besides correcting the CTD when used.