Smaller, smaller, smaller... I only say this since you asked and since you're still in the early planning stages.
As a training aid in a classroom setting for arty troops, it's undoubtedly technically and tactically correct. As Jerry said earlier, though, the rules of good composition don't always follow real life.
As a piece of visual art, it has too much going on over too large an area. What makes perfect sense to you (from your personal experience and knowledge of the subject) can be easily lost on other viewers as they look from one element to the next, perhaps never understanding the importance or significance of any of those elements.
Every one of your groups of "colored" guys is a story unto themselves, and all are more or less equal in visual weight and interest (and importance to the viewer). Therefore, you don't have a single focus or center of gravity. If everything is equally important, then there is no "most" important.
Paul's suggestion is worthy of consideration - breaking the entire scene down into a series of individual dioramas or vignettes, each with it's own main idea or story.
However, even if you continue with this larger composition, I'd still suggest reducing the size to eliminate as much of the open space as possible.
Don't try to do this by filling up that space, though, with small items or random figures, etc. Those things have their own visual weight and power to draw the viewer's attention. All that does is clutter up the dio and dilute the visual impact of the elements that you want the viewer to concentrate on. Eliminate excess space by rearranging the elements and reducing the overall size of the presentation.
You could, perhaps, carry this off if you placed the guns front and more or less center, facing the viewer with all of the other elements arranged in a crescent left to right and behind the guns and their crews. This places the guns (and the purpose of the operation) in the center where they are clearly the most important idea with the supporting elements creating a background and frame around this main idea. It would still be very large, but it would have more focus on a central (literally) idea. Relegate all the elements except the guns and crews to background and supporting cast and characters.
(This arrangement could be called radial concentric with a foreground emphasis rather than a axial interest.)
Doing this, I would also consider making the base oval in shape to eliminate the corners of a large rectangle (and the need to fill that space). This would allow you to "push" the guns forward of the elements on the left, rear and right, further reinforcing their prominence as the main element.
Facing the guns towards the viewer immediately grabs the viewer's attention since human nature makes us focus on objects and people that face us. (A slight turn to the left or right would be just as good and perhaps a little more candid and less "contrived" looking.) Bringing the guns into the center foreground intuitively gives them importance to the viewer. Action from the other elements can be directed towards the guns and the radially concentric composition helps to focus attention toward the center and the main idea (the guns).
My .02 (but only because you asked).
