How Tank Guns Work Home Tank Guns     Tank armament started as machine guns but this quickly changed to cannons of varying sizes so that the tank could deal with other armored targets and fortified positions. Most of today's modern tank guns are above 100mm, with 120mm being the most common. These guns are equipped with either a rifled or a smooth bore barrel and are housed in a rotating turret that allows the tank to engage targets at just about any orientation. These guns are also equipped with a bore evacuator that prevents toxic gasses from the fired shell from leaking into the crew compartment after the shot has been fired.   Here you can see a tank with its turret turned to the right. Also in about the middle of the barrel you can see an object that looks like a can, that is the bore evacuator.     Image source:http://battletanks.com/images/Leopard_1_Mod-1.jpg The Physics     When designing a tank gun, you have to first decide if you are going to use a rifled barrel or a smooth bore barrel. The difference being that the rifled barrel has rifling grooves cut into the barrel. The smooth bore barrel is just that, a smooth barrel with no rifling. The rifled barrel imparts a spin on the projectile by use of a series of grooves cut into the barrel. This in turn makes the projectile more accurate. A smooth barrel does not have rifling, but can shoot projectiles with more force because there is less friction on the projectile. Pictured below is a rifled 105mm tank cannon. The rifling in the picture is silver because it has been chrome plated. This is a common way of preventing corrosion in the bore. Image source:https://upload.wikimedia.org/wikipedia/commons/b/b2/105mm_tank_gun_Rifling.jpg         In the free body diagram below, I have drawn two gun barrels with a shot in them. The smooth bore will have less friction if both guns are the same size firing the same projectile because its coefficient of friction is less than that of the rifled barrel and we know that the force of friction is equal to the coefficient of friction times the normal force. This means that a larger propellant charge can be used on the shot with less risk of damage to the barrel or shot.        The bore evacuator works by trapping high pressure air from the shot as it leaves the barrel and then releasing the air into the low pressure zone that the moving shot creates, this then sucks out all of the toxic gasses allowing the crew to open the gun breech to load another shot without inhaling any harmful gas. Below I have drawn a diagram to better explain.