In the last 10 minutes of the movie, a character is shot by several other characters.
With each of these shots, an empty shell casing should have been ejected from the rifles (M1 carbine and M1). In addition, the empty brass would have made a noticeable "tink" sound as it hit the stone floor in the cellar.
When the "German" tank attacks the "V" shaped main gun support (used only when moving long distances and never in combat) is clearly visible. A few seconds later the support is missing. A second later it is back in place.
In every shot in which they appear, the "German" tanks have external travel locks on their main gun, including while they are firing. These braces are mainly applied when the tank is being transported by ship or train, and is to prevent the turret from shaking and damaging sensitive systems like aiming mechanisms. However, any such device would be removed long before the tank enters combat, as it makes aiming virtually impossible. That, and thanks to recoil, the act of firing the gun in such a situation would not only damage the lock, but also likely harm the very mechanisms the lock is supposed to help protect in the first place.