cc I reported this a while ago.
Same on the A8, either with outboard MG151/20s or 30mms.
You can fire, and stop, and fire again, but it doesn't ask "should this gun fire" until you fire the next time. So if you only have 1 round left in the MG/FF guns, but 300 in the inboard guns, you can keep firing all 4 guns until they're all dry.
Practically, you're not going to hold the trigger down for 30 seconds to really take full effect of this. I don't know about everyone, but I fire in bursts, which eventually cuts out the MG/FF.