Two nice examples of the joy of much dakka on Imperial vessels.
This first one is from Warhammer 40k: Firewarrior.
Get a chance to see an Imperial Fleet carrying out an Exterminatus the old fashioned way.
And second, we have the Lance Strike from the upcoming Space Marine game, starting at 1:05.
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So next time you are looking at the RT book and thinking about how powerful your ships are, these ought to be good reminders of what you can pull.