Quote Originally Posted by Soras Teva Gee View Post
Put aside for a moment Imperial ships blasting other 40k ships, do you have some other measure of their toughness. For example to Imperial ships ever dive into the atmosphere. We know Covenant ones do from among other things the Prophet of Regrets attack on New Mombasa, clearly through either hull or shields they can survive re-entry. Maybe Imperial ships simply lack the hover tech to do that, no problem, but that would be less arbitrary instance of toughness.
We can figure out how powerful their guns are from what they can do to things like planets. It takes far fewer ships and a shorter time for the Imperium's Lunar class cruisers to glass a planet than it does the Covenant, for instance. (Exterminatus on Typhon IV is carried out by five or so Lunar class cruisers, the fleet that glassed Reach had hundreds if not thousands of ships).

From instances like that where a common target is used we can tell that the firepower available to the most common ship of the line in the Imperium is vastly greater than those of the Covenant.