Ok, taking advice that a more traditional look might help, I re-worked some details.
Gone is the big nose gun, instead being replaced by a hangar bay for the marines and small craft. Honestly, I think it would be better this way as I cannot see where to put a hangar on a spinning ship in a parking orbit. It has to be on the long axis, and one end is all engines.
The ship got wider too, so the engine count is now seven instead of four, which helps add a centerline engine if you just want to run engines in groups and still keep it balanced.
I also changed the turrets to be more sci-fi, in this case small particle accelerator rings, and increased the count to six to keep the size of the ship up. It is shorter overall, though.
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---------- Post added at 11:34 PM ---------- Previous post was at 11:24 PM ----------
No need to fire explosives of any sort with a railgun. Either the mussle velocity is high enough that the pure kinetic energy of the projectile has enough destructive power of its own, in which case complicated warheads are unnecessary expensive, or you'd be better served with replacing the heavy railgun with a generous compliment of missiles. Since it's a scifi ship, I'd assume that huge railgun can punch quite some delta-v into its (seemingly quite large) projectiles, so no explosive on the world could really add more bang to their impact.
Actually, I would disagree. The impact of such a shot depends if you have speed. Firing a railgun in space is a good way to get velocity without spending fuel (electricity only), but as soon as it leaves the ship it is just a satellite like anything else. Kinetic kill weapons hit because of the tremendous speeds you can reach in space.
The problem is that they have to impact. Close doesn't count. Also, they are not as useful in a chase where the target is moving away from you, and worse the shot will last forever and could kill a satellite or civilian traffic later on, decades or centuries later.
A nuke would vaporise itself, even if you didn't hit anything. It would have to, as a dud nuke would be a serious mater if 35 years later somebody finds it and keeps it for themselves. Any nuke that misses should detonate anyway to keep it out of the wrong hands. You shouldn't need to worry about accuracy or space junk issues with a nuke.