Tanking out:
Bulletproof armor is usually behind guns because it faces a very real limitation; it has to be light. The average human can't wear armor heavier than fifty pounds and hope to retain any agility. Now, if you have someone super-strong who has no problem carrying around five thousand pounds...
What armor rating would you give to quarter-inch-thick hardened steel? That kind of armor for a human-sized combatant would weigh 200 pounds or so.
Given modern firearms to attack it with, a quarter inch of steel isn't bulletproof and is barely bullet resistant. A modern .308 rifle could penetrate that to a couple hundred yards, modern military ammo (like Soviet or Chinese ammo with mild steel cores to cut costs) can penetrate a quarter inch of steel at 500 yards. Modern armor piercing ammo could penetrate half an inch of steel armor plate at 300 yards and a quarter inch at double or triple that. A .50 BMG rifle, which is sure to be dragged out if armored trolls are on the offense, can easily penetrate an inch of steel plate at 200 yards with standard ammo.
What about the same armor from various superalloys?
In generate these alloys aren't used as armor because when a plate gets too rigid it shatters much easier, other much denser alloys are often softer than steel, like depleted Uranium. In general you don't end up saving a whole lot of weight, it just ends up thinner for the same protection. But at this point you're talking about 3/4 of a ton on a man sized suit to keep out anything smaller than a .50 BMG rifle with Armor Piercing ammunition.
It wouldn't be any thicker thus bulk would be no problem but it would weigh a bit over five hundred pounds and its strength would be three-five times that of steel. Still, five hundred pounds are nothing for someone with inhuman strength and some might or supernatural strength and no might.
And now think of a troll with comparable quality and quantity of armor.
The other problem is that weight grows exponentially with size. Double the size and you need eight times as much armor for the same protection; a 12' troll is going to need 6 tons of armor to keep out .50 rifle rounds and at that point someone's just going to treat it as the fae equivalent of a tank and hit it with a rocket. Or magic, how well do you think a troll can swim in quicksand while wearing 6 tons of metal armor?
Size does matter:
With a multi-barrel 112 inches long, total length of 19 ft and a total weigh close to 600 pounds, the GAU-8/A is a BFG. It fires 30mm antitank bullets at a rate of 70 rounds per second and can turn tanks into swiss cheese. A superhumanly strong guy or gal that is still human-sized might have some trouble with its bulk but not its weight and a troll or ogre would wield it like we do an assault rifle. Since we have seen Gruffs use machineguns already, why not inhumanly strong opponents with that kind of heavy weaponry? They even have glamours that can hide their size and form until it's too late.
So, what weapon rating would you give the GAU-8/A ?
The GAU-8 is a tad more than 600 pounds. Fully assembled it weighs just over 4,000 pounds with ammunition and that's without a power supply to rotate the barrels. It's also a tad unwieldy, it's longer than a car and the ammo drum is about as big around as a volkswagen beetle. You don't casually pick one up and swing it around without a plane sized mount for it. For the same reason that if you could lift 50,000 tons you couldn't pick up a building and throw it - you'd just come away with two handfuls of concrete and the building would still be intact.
As for weapon rating? The FATE system doesn't really model armor or damage above a personal scale, it just becomes a plot device at that point. I've got better systems to use if a GAU-8 Avenger Cannon becomes a standard ranged weapon I need to stat out.
Also:
An A-10 Thunderbolt has to fire their guns in short bursts or the recoil will knock them out of the sky.
Complete myth I'm afraid, there isn't even enough recoil on that thing to even slow it down a few mph when it fires. The reason it fires in short bursts is to avoid burning out the barrels through overheating.
