A 19,999 J Meson Needle cruiser with J4/M6/Agi 6, Model 9/fib computer, and #9 screen can have a total of 1 armor, no weapons, no vehicles. It has a Power Plant 13, and costs 16867MCr.
A Rider model (forgoing the J Drive and fuel) is 6423 DT w/15 Armor. It costs: 8389MCr.
The 74,999 T Meson cruiser has 3 armor, and room for 10 50t bays for missiles (no real power for anything else and keep Agi 6).
it costs 56941MCr.
You can cram a J Meson in to 15 Armor, Agi 6 boat -- but it comes out at around 210,000Dt.
The unarmored ships would be burned to a cinder by a bunch of smaller ships with nothing but BLasers and missiles. They may not "kill" the ship, but they'd knock that big gun down to scrap in no time, making the ship combat ineffective.
The 15 Armor Rider is immune to anything but Spinals, initially, but once hit by a spinal, the armor damage would make them vulnerable to turret fire, which could quickly reduce the gun.