The Apocalypse offers a few things that the Armageddon and Abaddon do not.
First is cap stability. Its 50% cap bonus with lasers combined with its superior capacitor strength allows this beast to permanently fire its laser turrets without wasting precious mid slots and low slots on capacitor modules. This allows the pilot to fit additional tank/dps/whatever modules instead.
Secondly and most importantly, the Apocalypse's range bonus is what truly sets it apart from the other Amarr battleship options. Simply put, greater range makes it the superior Amarr sniper.
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