Is it a AAA Battery? I could have sworn it was an Artillery Cannon for ground based assault..

Though... there are AAC Batteries so I guess it makes sense. :shrug:
My guess is because the Hoover Dam IS a High Value Target so it would make sense to have defences for Land, Air, and Sea.
Well, it doesn't appear to be a perfect match to any real-life artillery piece I've ever seen, but it appears to have borrowed a lot of its appearance from the US 155mm Long Tom howitzer, which was intended for lobbing shells long distances at land targets, not for AAA.
http://en.wikipedia.org/wiki/155_mm_Long_Tom
It kinda looks like a design cross between the Long Tom, and a german railroad gun from the WWII era- example: http://floydworx.deviantart.com/art/Dora-railway-artillery-50786415
A bit bigger in size than the Long Tom, and much smaller in size then the Dora, but with a lot of design elements from both. And definitely not an AAA gun.
(Although it may be purposed that way in the game, who knows... but the thought of an advanced military in the year 2077 still using slow-firing huge-bore artillery shells for AAA purposes, just cracks me completely the hell up- we stopped doing that over 40 years ago. Jets and missiles made them utterly obsolete.)