The tank commander will ask for a specific type of shell. "HE" is high explosive. "AP" is armor piercing. "Willy Pete" is white phosphorus.

The loader will retrieve this round from the ammo racks in the turret and load it into the tank's main gun. He will then yell "Up!" to let the tank commander and gunner know there is a live round loaded.

The commander will give the order to fire "Fire!" and the gunner will yell "On the way!" so that the loader and commander know to stay clear of the recoil path. It is possible for the commander to let the gunner control the rate of fire, especially if he is also a platoon leader and has to coordinate the battle. The loader and gunner will still trade "Up!" and "On the Way!" to communicate with each other and keep everyone out of the recoil mechanism.

John P. Irwin's Another River, Another Town is a good primary account of an American tank gunner in the late stages of WWII.

Does he really shout "one"? I can only recall the phrase "on the way!" being used by his character in this respect, and that's already been explained.

I found several things relating to tank drills. This post, seems to indicate that the gunner will should "ON THE WAY" or "FIRING" when he goes to fire. Further down the page, it is mentioned that different countries have different procedures.

This manual for the M4 Sherman makes no mention of the gunner shouting anything.

Here is a modern day video of a Bradley and Abrams crew. Both gunners say "ON THE WAY" when they fire.

While we're here, is the film any good? From an AskHistorians perspective, I mean.

Re: the discarding sabot, this is used in virtually all tank guns now, but in WWII most antitank shells were just a bullet-shaped, super-hard solid projectile. Everybody started the war with little guns (20-40mm) firing low-velocity, and the Germans were the first to increase caliber, velocity, and barrel length (which adds to accuracy, range, and velocity).

The US excelled in engine performance and reliability, but stubbornly lagged behind in gun development and doctrine (still had a separate force of lightly-armored 'tank destroyers').

The British doggedly clung to outdated doctrine (slow, heavy tanks for infantry support, light, fast tanks for tank battles) until they adopted the American tank wholesale, and then improved it with a long-barreled, high-velocity gun that could actually kill German tanks.

The Germans developed several excellent competing tank designs, each of which had some major flaw, usually in the propulsion and/or production. At the end of the war they had produced a large number of mediocre PzKw IV medium tanks and tiny number of the Tiger and Panther super-tanks.

The Russians built many different models, but eventually settled on a superb tank (fast engine, good armor, great gun) and built 35,000 of them.