Harry adds sandbags around the back of his cannon to hold it in place, yet when he is firing the cannon, the sandbags are nowhere to be seen.
Harry piles up six shells next to his cannon, yet in all of the subsequent shots of the cannon, the shells are gone.
The heavy-artillery implements that Pinky uses are obviously vintage weapons that lack any sort of modern-day automatic-loading/remote-firing mechanisms, so they would have to be operated manually; the Panther would not be able to continuously fire the swivel-gun and the tank-cannon at the same time, the way he does in the movie.
The swivel-gun that Pinky first uses would not be able to fire without an ammo-belt.
The loading-breech on Harry's cannon is wide open in many of the shots of him firing the cannon.
Harry was facing a constant barrage of machine gun fire and artillery shells, and so he was being forced to just cower behind the protective shield of his cannon. So there would have been no way he could have gone over to his pile of shells to reload his cannon.