You obviously didn't read the wiki description ty, or look at the picture in detail for that matter.
It may share the size of an AT-AT, but it's a shuttle, a shuttle is about twice as large as a fighter, and a fighter is about 1/2 the size of an AT-AT. Look at the trooper standing next to it. It is no bigger than a sentinel. Also, quote from the description about those things you call guns: "Two long booms of unspecified purpose extended forward of the main hull.". They never said they were guns. It's only weapons are double turbolasers on either side. This picture shows the ship as landed. When it's in the air, that huge landing gear coming out the bottom is inside, and it's just the top section of the ship you see. It's command pod is like that of a gallofree, it's a little pod on top raised above the ship.

Curses, foiled again!

My answer to all of this is why mine the New Republic era for content when we have barely scratched the surface of the period featured in the original movies? I have stated this countless times before, but since people seem to have short memories here it is one last time.

First Strike is currently concentrating on the Galactic Civil War period, or the time covered in and around the original trilogy of movies. We will be sticking to this content except in circumstances where no movie equivalent exists for the asset we wish to create, such as infantry based rocket launchers, tanks and some lighter class capital ships for example. In these and only these instances will we delve into EU (Expanded Universe) sources to find a suitable solution such as the HH-15 Portable Missile Launcher, Carrack cruiser or T1-B Hovertank. We may also use EU sources to come up with interesting locales for maps other than the battlefields or locations seen in the movies.

Should this stance change believe me you'll be the first to know.

Alright so if theres no rebel shuttle in GCW we'll find something cool out of EU.  Works for me. I just hope that we dont end up with rebels using imperial shuttles. >_<

Curses, foiled again!