I just experimented with the Junker: gat top, mortar and a gat up front.
At closer range the gats can just barely hit the target (mortar was out of arc) But the pilot has zero margin of error and the gunners no arc to compensate for ship movements. On medium range at least the gunners have a respectable arc to work with as long as the pilot is accurate with his positioning.
I think a gat up front can work if you have an able and chatty crew who also know what the pilot wants to achieve. I can imagine this being a charging pyra killer. Double gat on the approaching target and then just shred them to pieces with medium/close range mortar.