Sure, this is a point of semantics, but it's important to remember when we're phrasing how things work in this game - especially when teaching new players - that all small and large ships have a primary weapon arc (or firing arc). Some have an auxiliary firing arc. Some have a turret primary weapon. But nothing in this game has a 360-degree firing arc.
This cannot be repeated enough: NOTHING HAS A 360-DEGREE FIRING ARC.
It would either invalidate or over-power so many upgrades if any ship were to have a 360-degree primary arc. A turret allows you to attack outside of the weapon arc, in addition to attacking inside the arc as well. Every turret secondary weapon has the qualifier on the card of: "Attack: Attack 1 ship (even a ship outside your firing arc)."
From the Rules Reference Guide, Page 19
TURRET PRIMARY WEAPON
Some ships use a turret as their primary weapon.
Each of these ships has a unique primary weapon
icon, and each ship token shows a circular red arrow
around the token’s center as a reminder. When
attacking with a turret primary weapon, a ship may
target an enemy ship inside or outside its firing arc.
• A ship with a turret primary weapon still has a
printed firing arc that is not 360º.
• When attacking with a secondary weapon, the
ship must still target an enemy ship inside its
firing arc (unless specified otherwise on its
Upgrade card).
For example, if it was true that a ship did have a 360-degree firing arc, then Tactician would work ALL THE WAY around the ship in a giant Range 2 ring - "After you perform an attack against a ship inside your firing arc at Range 2, that ship receives 1 stress token." Then any attack, with a true 360-degree firing arc would always be in arc, and anything at range 2 in any direction would be subject to the card ability.
Autothrusters would rarely work as you would never be outside of the firing arc - "When defending, if you are beyond Range 2 or outside the attacker's firing arc..." Any ship with a true 360-degree firing arc would nullify Autothrusters from ever being outside of the arc.
It's a slight, but incredibly important, distinction for this game to function properly.

