Liners should have the Gunboat/Transport turret hybrid turrets.
But, in return, they should have 3000~ cargo Maximum.
And they shouldn't even comparable to the Advanced Train or Whale.
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That or, they should cost 200 million~ each and be left unchanged in every aspect.
Liner turrets would use up, unnecessarily, the class 6 variant of turrets which could probably be better used in the future when we get a new shipclass worthy of their own guns, such as, for example, if the Battlecruiser gets specialised guns, of which there is a much larger case for this than liners getting their own turrets. They are transports, as stated earlier. You can't have armaments protection, cruiser armour and high cargo holds for the same amount of cash as an advanced train.
(Insert model here) Liner: High cargo hold (behind by about 500-1000), Average (cruiser standard) armour, very useful guns.
And both these transports are massive, and cost the same amount. No, the Liner isn't the same size as the Osiris; no, it doesn't turn like a battleship; and yes, they are way to cost effective, with their shields too.
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Lucky me, I stockpiled 3 Advanced Transport shields.