The issue with the MG FF in-game is that it uses shells that were only later used by the MG FF/M.
Historically the MG FF only used HE-T and Tracer shells. Other types of ammunition only become available when the MG FF was already replaced by the MG FF/M which was able to fire the lighter Mineshells.
https://community.gaijin.net/issues/p/warthunder/i/cVQRAK08JYTS