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

We've long speculated about variable MAC yields, at least since the Spirit of Fire apparently didn't cause nuclear winters from orbit in Halo Wars, and the MAC strikes in Halo 3 and Reach similarly didn't devastate large portions of the planet. 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 powerful frigate light coilguns are versus heavy coilguns on destroyers and cruisers, the difference between the Infinity's guns and a Super-MAC platform, etc? Fans have wanted to know more about MACs for a long while - the very fact that railguns produce recoil still comes as a shock to some, who assume otherwise.