[Rollo]

I see. I have also tried a full 360° arc, but the game choked on that.

Try an arc from 22.5 to 337.5 or 202.5 to 157.5. The game handles arcs in 22.5° increments without problems and that only leaves a small blind spot in the front or rear.
Also you need to change the entry for the appropriate firing point in the SpaceObjects_Xfiles.txt file. Just changing the slot is not enough. In fact, IIRC, the setting in SpaceObjects_XFiles.txt is the one that counts. The entry in the slot file is kinda redundant.