Thanks for the detailed explanation, bradmick.
In the meantime, I found further explanations in "Chuck's AH-64 Guide, part 14, offence weapons & armament". Interesting for me: after solving my problem by finding the right key, the "coincidence" message still exists, without any consequences when firing the gun. Also shows the Cp/G´s IHADSS "PGUN", which means to me that the pilot, not Cp/G, actually is in command.
Anyway, all is fine for now and thanks to all, who tried to help👍
Trigger's post in "Coincidence" when deploying weapons was marked as the answer