By wiki (aka Lore), the Spirit of Fire had:
- Magnetic Accelerator Cannon
- 50mm point defense guns (Estimated 52)
- Deck Guns (22 or more)
- ATAF Missile Turrets
- HAVOK Tactical Nuclear Weapon
So, the answer (I suppose) is: The Phoenix-class ass-ship has one MAC.
Now, we must consider that we are talking about a ship that was by every means a one-of-a-kind. We know of only one Phoenix-class Refitted Colony Ship, the Spirit of Fire. This can bring to some conclusions:
1- its role in the mod could be changed to be a hero unit (and its place taken by some other carriers);
2- they are refitted versions, and are very likely to be not mass-produced; hence, one ship could be completely different from another of the same class (the SoF, for example, had its engines replaced by those of a Destroyer to increase thrust power); hence, categorizing them as a "Class" could be very wrong --->see point n°1.
But for the sake of this analization, let's keep the Phoenix-Class Assault Ship as we know it.
We must also consider that UNSC in the mod excels in Alpha damage: this means that if we give a single ship the ability to fire MAC rounds at multiple targets, the ship could become possibly very, very overpowered. Now, we could reduce the MAC damage as suggested, but there is the very likely chance that, given the already-low DPS the Phoenix's MAC has, the ship would become completely useless against early-game Covenant (the time of the game where Covenant excels), because MACs are the primary effective weapon of UNSC's alpha strike damage.
The MAC damage is already quite low. Lowering it means no utility against Covenant (and UNSC alike, honestly...it isn't that much of a power ship) forces. No utility means no one would build it. If no one builds it then it has no role in the mod. If it has no role in the mod, then we must cut it out. But we want to keep it in, so there's a problem with lowering the MAC damage.
Now I don't know what you mean by muzzle, so I don't know what to say about it.
"Humanity already tested it's coilguns in 20-21st centuries. I doubt that in 26 century it would be a problem for small versions."
Spoilering this out because it gets Off-Topic
Last but by no means the least, we have to consider that in Halo: Combat Evolved, in the level The Maw, you run through the PoA for approximately 4 kilometers. On a ship that's not even 1 kilometer long.
Things change during the course of time, and as the universe evolves, developers are very likely to retcon many things.
Sometimes you sacrifice Lore to Gameplay.