Ok, Nuncio, so we at least have eliminated one possible source of the problem.
Question 1 : does the problem occur with each and any weapon (including melee weapons) ?
Question 2 : if you actually build a standalone game from a NEWLY CREATED map, does the standalone game show the same (mis-)behaviour ?
If so, I suggest you make a small map with only the player and one or two weapons, build the standalone and give me download link to the
standalone game, PLUS a copy of the map only (you know I know how to analyse them) , too, please.
Alternative orders

: Please ...
* rename your current GameGuru Top level Folder - in order to get almost all you have now out of the way easily.
* then have Game Guru installed by steam from scratch to the original destination (probably an overnight job, given the number of models you own)
* do NOT add anything (such as free or paid models/scripts/skies/sounds/anything from other sources) that does not come down from steam with the installation
* Do NOT copy maps or anything from the renamed maps folder of our heretofore GameGuru folders under their new "root" directory.
* Do NOT rearrange/rename directories within the new Game Guru
* CREATE a small map with a few weapons, run it in testmode and report the results.
* Also build a standalone game employing this map and report the results.
* If the map runs ok in the "new" Game Guru, COPY the map to the respective path in the renamed old GameGuru
* run the "old" GameGuru.exe in the renamed old GG Path directly by doubleclicking on it
* load the map
* if the weapon files had been relocated from their standard (relative ) path in the "old" GG directory structure by you, GG won't find them and let you know.
* in that case, REPLICATE the standard path for the weapon from the new GG in the old GG directory and copy only the files pertaining to that weapon
from "new" to old-reinstitued-standard-weapons path and see whether "old" GG can use it now (it should!).
* let us know the results.
good luck
Lives of great men all remind us we may make our lives sublime