Videos supplied are from a Training film of the US Army, concerning Tank engagement procedure on the M551, but these are similar for all Tanks. I know that the procedure detailed above was used in the 70s/80s in the German Army, and I think Arma can take an Update.
http://youtu.be/4gANMzM6p4M?t=1m22s <- IRL procedure for engaging with MG (This should not have changed much, the procedure has only been supplemented by modern sensors.)
http://youtu.be/4gANMzM6p4M?t=2m11s <- Engaging with main gun. Note that Gunner continues to engage target until told by commander to explicitly seize engaging.
Secondary request would be a Commander Turret override (Possibly by holding right mouse on target and clicking left mouse, causing the turret to be forced onto the target.) Not as important as Firing procedure changes, however.