TANK ATTEMPT

usual a weapon has a very serious set of mechanisms that attach it to the hull or control it... in a lot of designs these are neatly hidden inside the hull, or turret and the only thing showing are the barrels....

also straight up and down surfaces are not a good idea on armored vehicles... they attract high velocity projectiles and are an enemy gunners dream.... slowped angles on turrets are practical as well as look a lot more lethal. 

the smaller weapons on the hull of a vehicle aren't very common ( more a practical consideration than anything...no reason not to use them if you like the way they look)... usually they move those to either co-axial...( mounted along side the main guns) or mounted on top of the turret where one or two guns can cover a 360 degree arc and engage airborne targets without the turret getting in the way.

Most military vehicles have to survive being reviewed by bean counters...so they look at them and go..why two guns..cant you make due with one..and why all those little guns you don't need that many little guns.....but once again if you like the way they look to hell with the bean counters.