I've seen plenty of movies where a mortar shell is dropped into the tube, and then goes shooting back out. What exactly is going on there? Does the shell explode at the bottom of the tube and then go flying out? Where is the propellant in the process?
The propellant is in the bottom half of the shell that is dropped into the tube.
The most basic tube you can imagine is a steel tube that is closed on one side. On the closed side there is a nail pointing into the tube.
When the tube is only slightly larger than the the shell, the following happens when you drop the shell into the tube:
-the shell exerts pressure on the air already in the tube.
-The air tries to exit the tube, which it only can around the shell. Because the shell has almost the same diameter as the tube, this process takes some time. That is a good thing, because it allows you to duck for cover.
-When the shell reaches the bottom of the tube, the nail pierces the shell and ignites the propellant (through a chemical reaction, which additionally delays the explosion)
-The propellant expands rapidly/explodes inside the barrel. There is only one way out of the tube for all the pressurized gas. The shell is in the way. Off we go.
Very, very basic explanation of what happens inside the tube, but I think you get the idea.
A mortar is a high angle of fire, low velocity weapon. It has three primary components: the mortar tube, the base, and the bipod.
The components of the U.S. M2 mortar weighs 42 pounds. Its tube, or barrel, is 29 inches long. It launches a 3 pound projectile 2000 yards at a sustained rate of fire of 18 rounds per minute.
The components of the M1, from which the M2 was derived, weight 136 pounds, with the three main parts each weighing around 40 pounds. Its tube is 47 inches long. While the M2 can be carried by one individual, the M1 is usually carried by three individuals. It launches a 10 pound projectile 3300 yards at a sustained rate of fire of also 18 rounds per minute.
Mortar rounds are fin stabilized, with the fins shaped so that there is a characteristic whistling sound as mortar rounds fall. I can personally attest to the negative psychological effect of that whistling during the 30-45 seconds the round is dropping into the target area.
There is a primer at the base of the round which looks very like a typical primer in the base of a bullet cartridge (originally, shotgun shells were used). The mortar tube itself has a fixed firing pin located in its base. When the round is dropped down the tube the primer impacts the fixed firing pin, the propellant is ignited, and the round is launched at approximately 600 feet per second. Range can be increased by snapping small propellant packets to the base of the mortar round before dropping it into the tube.
Mortar rounds are of three common types: smoke or white phosphorus rounds used for marking or hiding positions, illumination rounds used at night to light up the battlefield, and high explosive rounds used against troops or unarmored vehicles in the open.
As I mentioned earlier, U.S. forces use 60 mm and 81 mm mortars. Eastern Bloc and Communist forces use 61 mm and 82 mm. It was very common during the Vietnam War for Vietcong to capture American 60mm mortar rounds and use them in their 61 mm mortar tubes, as close tolerances are not important in a mortar tube. They also used American 81 mm mortar rounds in their 82 mm mortar tubes for the same reason.
There are many other types of mortars, including some with nuclear projectiles. Mortars also lend themselves to being easily constructed, and can use anything from compressed air to cow dung to launch low velocity projectiles. The Irish Republican Army, the Viet Cong, and numerous other insurgent fighters used handmade mortars and rounds (and still do).
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