I've finally made a custom weapon.

It's a Torpedo...

Tutorial:

1. Get a detacher (disable it's activation group)

2. Get a weapon (missile, bomb, rocket (not rocket pod), torpedo) and attach it to the bottom of the detacher.

3. Make a custom weapon fuselage thing. (Calculate Drag;false, Mass;0, DisableAircraftCollisions;True)

4. Attach the custom weapon to the back of the detacher.

5. Move it via XML to the position of your weapon.

6. Put the detacher into the weapon (hidden) and reattach the detacher on the Hard point.