The medic of the team carries a weapon all the time. According to the Geneva Convention, military medics are noncombatants. They could/can be armed for protection of themselves or the wounded..
When Southern and Coolidge meet Lt Smith for the first time they salute him even though they're in a combat zone. Saluting is not done in combat zones because it can signal to the enemy who and where the officers are.
When the Americans enter their objective town, the Germans are currying ammunition crates.
The crates are labeled with American stencils reading "CARTRIDGES FIR WEAPOMS, INERT PROJECTICLE".
At the beginning after the three soldiers meet up, one soldier changes the magazine in his M3 Grease gun, it is obvious the magazine is for an airsoft weapon and has the small opening for the BBs and no brass .45 cartridges are seen as they would if it was a real weapon.
None of the weapons fired eject spent casings.
At 25:49 in movie, you can see 1 (or 2) modern security cameras in the top right window.
The US soldiers keep telling that their mission is to take the French resistance man to Bastogne.
Bastogne is located in Belgium and was never part of the Normandy campaign.