The above explanation, using the equation provided as well as the nuclear explosion simulation maps model is very accurate and in all honesty one of the best available representations of what a mac round would realistically do to the surrounding landmass. Of course the explanation that it is not a nuclear based weapon and it would act more like a asteroid impact is correct as well. the subsequent dust cloud would have devastating effects on the planet but we also need to look at other possible sources.
Inside the book Onyx, I believe there was a moment in which blue team was assaulting a covenant position, now I don't remember exactly where the mac round they ordered was coming from, whether it was a conventional ship mounted mac gun using a 600 ton tungsten round or if it was a orbital super-mac using a 3000 ton tungsten round. The massive variance between the two ultimately change the probable outcome.
The specifics behind it where that the mac shot would pass through the docked ship and impact the ground, if I remember correctly, and I can always check later and update, or post another post with the specifics, they explained that if the shot was used as is, it would impact the planet and actually preform as a "planet Killer" class weapon, in such as way as to basically destroy the tectonic plates underneath and rendering the planet uninhabitable. I am unsure of this 100% but I remember that being somewhat the case, and that was the reason they used a Shiva Tactical nuke underneath detonated at the right time to act as an opposite force to bleed off the impact forces of the mac round.
Based on a 3000 ton tungsten round impacting the ground your looking at something like what is provided in the post above. regardless of what we're looking at the "airblast" radius provided above is inaccurate due to the fact that it is based off a exploded ordinance at sea level, that isn't in a solid form upon "impact", the actual shell itself impacting into the earth would have devastating consequences on the landmass let alone the uprooted earth being thrown into the air and realistically the odds are high that the world would be faced with a "Nuclear Fallout"/"Iceage" level event killing all life on the planet. if the subsequent tectonic and landmass changes didn't kill you first, the freezing of most if not all of the planet, and the "ash" cloud of earth raining down on you would, realistically, you'd die from suffocation, possibly magma, or as the last thing should you survive those, freezing on a now barren frozen wasteland.
At the end of it all, the mac weapon should be considered a planet killer, and at worst a super weapon of epic proportions, but hey, its a pretty good weapon when killing aliens.