As I told Manny in his thread on the 150mm gun, an artillery position is the most complicated positions to show. Even with a small gun like this, there will be live ammo, ammo crates, open crates, fired brass, section chest, commuication (phone or walki-talki), ramming staffs, and personal weapons & equipment stacked.
Even a hasty position which has been fired from will have much of the above. I was planning on all this for the M3.
If it is a matter of staying small, in the WWII era you have the M3 105mm, The M2 105mm, kit from Italeri, little bit bigger than the M3. Also the M1A1 75mm pack which various resin kits, Tamiya old metal kit and a new styrene kit coming from a new company called Vision.
105mm ammunition is avalable from Verlinden. The also made 75mm pack ammo. That kit is discontinued but shows up now and then on ebay. Crates for both can be found from Verlinden and others. On all of these I would recommend you buy one and cast as much more as you need.
Commo equipment is availabe from Warrior and Verlinden. Ramming staffs are scratch from styrene rod.
The M3 kit goes for about $25 from CGM. You will pay about the same for a 75mm pack resin kit, most of which are copies of the Tamiya metal kit. The Tamiya kit on ebay will be about $50+. The Italeri M2 105mm will be in the range of $15-20.
Hope this helps.
Rounds Complete!!