Ranges on cannons/rockets are deliberately set this way as a balance consideration, and to encourage building diverse defenses, with multiple turret types to cover each other. I tested this with deathworld runs and rampant runs, and they perform fine.
Restricted arcs also meant turrets don't blow up your own walls with explosive ordnance. I saw quite a few rocket/cannon mods that suck, because 360 arcs mean they lob explosives at biters right next to walls, blowing both the enemy and your defences. By the way, huge minimal range on backline rocket turrets mean you can actually use nukes in them for defense, and they won't blow up your own stuff (when placed smartly that is).
240 arc in the item tooltip for backline HMG is outdated, sorry about that, forgot to update it.