The rules, again
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1. Any armored vehicle, Allied or Axis, that was in action between the Caen - Cherbourg road and the coast between June 6th and June 9th 1944 in any scale will qualify. Any vehicle used to storm the beaches - ex. LCVP, LCM, DUKW - or diorama of the D-Day landings will also be allowed. The same goes for the airborne landings.
2. Markings must be of any of the divisions - US, British, Canadian or German - present at the aforementioned time frame. Wading gear isn't mandatory, but strongly encouraged. The same goes for the Sherman DD, Sherman Crab, Churchill AVRE and other "Funnies" of the 79th Armored Division.
3. If the unit of the model you're building was in Normandy at the time, and it's been know that this unit was equipped with the model you're building, it's assumed that the model you're building qualifies. The model must however have the decals of the qualifying unit.
4. The model must be armored, so no jeeps and trucks or other soft skins.
Artillery does have some armor.
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ok, you could argue the same about an up armoured jeep off course. It's just, there were so maaaaaany jeeps and trucks that it wouldn't justify this campaign.
So, for the artillery question, it's ok, as long as you make your dio typically Norman, meaning thus you set in a bunker type environment, or being assaulted by airborne or seaborne troops, or anything you can think of. I think this way it sticks to the theme, but gives you a lot of freedom in your model.
Dave