If "Certain weapons" are scar, lr300, and other ones that use that upgrade schematic (don't know anymore besides those 2, but there are lots i guess) use the "base game" w_lr300_up.ltx. I did it and now i can upgrade both weapons mentioned before. Try it out and don't forget to backup your original w_lr300_up.ltx file.
if the spas12 is the problem then use the "Base game" w_spas12_up.ltx. I personally didn't find that weapon so i'm unable to check if game crashes when upgrading, but i saw a few comments that says it crashes too. Don't forget to backup w_spas12_up.ltx too.
Hope it helps.
EDIT: Found one spas, the game crashed with the "STCoP Compatibility" file.
Change it for the "Base game" file, worked like a charm.
JoelLxc
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