When accessing weapons slot directly from the hangar or empty ship slots, sometimes some modules of the highest rank 15 aren’t available! You can only buy rank 14, but not rank 15!
Spotted and reported missing weapons/modules so far:
rank 15 coilgun
rank 15 meson cannon
destroyer coilgun rank 15 (Missing rank 15 Mark 1 Coilgun variant)
destroyer meson cannon rank 15 (Missing rank 15 Mark 1 Meson Cannon variant)
Rank 15 Halo Launcher is not bugged. You can buy rank 15 Mark 1.
If I will find any more such cases, I will update this bug report.
How to replicate this bug:
Equip a rank 14 Sirius, Tyrant or Vigilant destroyer
Select the weapon slot in the hangar on the ship directly
Try to search and buy for Rank 15 destroyer weapon Mark 1