Alright, so there has been a couple vids on this Easter egg Mac cannon and it's really interesting but you can find a whole bunch of Mac cannons all in one place by just loading up the Darkblade strike and heading to the asylum and once your there look to your left and just check out all the Mac cannons in the scenery. So the hidden one isn't that much of an Easter egg since there is already some in the game but A+ for effort on the guy who found the original one in the raid. Check link for original vid