[F4U-1_Corsair]

ok, a simple way to find whether or not you can fire over sand bags, entrenchments, etc, look for the center of rotation of the barrel. if it is above/in front the sandbags, then it will fire fine. you can use this to get MG's to fire out of trenches by placing them so that the gun mini- tripod center is barely ahead of the trench. You can get the gun emplacement feel in 50 cal AA guns, by using 5+ streight sandbags to form the defence instead of 1 circle. you can take up the extra space with ammo boxes, and add another soldier object to look like he is grabbing ammo (again, out of any areas the gun will fire though) to add a touch of life to the scene, which really completes the image when you are flying over a troop concentration etc.