Okay, we had the scene at the end of ONI: Sword Base, but that was likely a low-power shot.
If I recall correctly, a super MAC has a yield in the gigatons, though it may have been retconned. That's more than a hundred times the energy of the Tsar Bomba, which was already unimaginably powerful. I'm guessing it could potentially be more effective at glassing a planet than the covenant energy projectors, and possibly even causing earthquakes and tectonic disturbances(I am not a geologist).
[Edited on 07.28.2012 5:52 AM PDT]
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There cant be nuclear winter... [quote][b]Posted by:[/b] LoopyA muffin 3 It would probably sterilise the planet in the blast and the the resulting nuclear winter and may cause tectonic disturbances resulting in volcanic eruptions and possibly tsunamis if the oceans had Not mostly evaporated by that time. E.g. No good :( [/quote]