: As I understand, according to an editor, the repeated description of MAC
: rounds penetrating through Covenant ships means that they are not
: hypervelocity impacts - which means that for a tungsten round, they must
: be travelling less than 1.7 km/s, or they would atomise on impact, rather
: than penetrate right through.

Not all hypervelocity impacts are created equal.

That is a navy railgun piercing multiple sheets of steel, each one a quarter inch thick if I recall correctly. Compare to orbital debris, which can be stopped by a thin sheet of metal spaced a few feet from a spacecraft's hull.

Whether the projectile disintegrates or carries on through the target depends on the target's sectional density, the projectile's sectional density, and its material composition. What is the speed of sound in a Covenant warship? How do shockwaves travel through the projectile? How does it fracture? How does the plasma from the obliterated outer layers penetrate deeper into the projectile.

Most hypervelocity impact research (i.e. what people cite when they say that MAC rounds should behave like that) is designed to simulate meteorite impacts on moons and spacecraft and innocent little planets with beautiful oceans and sapient megafauna. It's a useful approximation to a degree, but it's only an approximation.

Anyhow, overpenetration of MAC rounds and hitting multiple targets is indicative of three things. The first is that the UNSC's MAC rounds are actually inefficient for killing warships. It's just like a regular bullet fired from a rifle; if it passes clean through the target, there was energy in the bullet that wasn't transferred to the target. A better bullet design would have it fragment, tumble, or otherwise disintegrate inside the target. A case like the Covenant warship coming after the Commonwealth after a MAC round punched clean through it would not happen.

Of course, it's possible that there are multiple designs of MAC rounds, some optimized to kill ships and others optimized to penetrate atmospheres and mountain ranges. It's also possible that shields are so durable that a MAC round that merely breaches a warship shield will punch clean through an unshielded warship.

The third problem is that Covenant warships are durable. Really freaking durable. You've seen the flames that dance around reentry capsules and space shuttles as they glide back into the Earth's atmosphere? Turns out, that's compressive heating. Those objects move too fast for air to flow around them, and instead air piles up and compresses in front of them. PV=NrT, so compressed air heats up until it turns into plasma.

MAC rounds are beyond compressive heating. Air and bulkheads and layers of MAC rounds are going to be flashing into plasma. The effect is pretty much akin to a nuclear explosion inside a Covenant warship. And these warships have been known to survive said impacts along with their crews.

The final problem is that Covenant fleet formations suck. Seriously. They've got all the space in the solar system and they're bunching up close enough for multiple ships to be taken out by a single MAC round? They could be spaced dozens of kilometers apart and still support each other, with a negligible risk of collision or multiple casualties.

: My concern was that, while a plausible
: explanation, I would prefer it to come explicitly from a reliable source,
: either in-universe, or from an authority - Catalog, or Grim - since, this
: being a fictional universe set in the distant future, they may have just
: found a way to offset hypervelocity impacts.

It's worth noting that hypervelocity impacts in Halo are weird to begin with. Take Contact Harvest for example, when Johnson shoots an Insurrectionist with a Stanchion Rifle. The bullet passes through a concrete highrise like its not even there, but reduces the man to a quivering pile of hamburger and bone meal.

So, let's get this straight. A bullet perforates concrete floors without vaporizing or performing any work on the concrete, but practically explodes after passing through a few decimeters of meat? That's not how ballistics work. At all.

: Or even more simply, it was
: written/shown that was for creative license, because it looks cool, not
: because it's accurate.

Boring.

: I don't suppose it would be possible to address
: some of our questions about MAC technology - this issue for one, how ships
: offset the sudden recoil during combat,

How is the MAC cannon mounted in the frigate?
Do frigates still weigh a few thousand tons, or have we pulled a Weber and retconned it up to a more sensible number?
What's the weight ratio between the MAC round and the MAC cannon? Between the MAC cannon and the warship?

Second question: How fast can UNSC warships cross a solar system?

If UNSC ships can get from Neptune's orbit to Earth's orbit in a single day with sublight drives, they are capable of a sustained 4.8km/s acceleration. Such a warship could possibly flare its engines to offset the recoil. And if inertial dampening systems (Hint: If you have artificial gravity, this shouldn't be too hard) are installed aboard the ship, crewmen won't notice that the ship has suddenly decelerated at a rate that should have splattered them against the bulkheads.