totally different types of breech. A metal case holds an integral primer to initiate the propellant and provides the gas seal to prevent the gases leaking out of the breech; this is called obturation. With bagged charges, the breech itself provides obturation and holds the primer. In either case, the primer is usually percussion, but electrical is also used, and laser ignition is emerging. Modern 155 mm guns have a primer magazine fitted to their breech. Artillery ammunition has four classifications according to use: Service: ammunition used in live fire training or for wartime use in a combat zone. Also known as "warshot" ammunition. Practice: Ammunition with a non- or minimally-explosive projectile that mimics the characteristics (range, accuracy) of live rounds for use under training conditions. Practice artillery ammunition often utilizes a colored-smoke-generating bursting charge for marking purposes in place of the normal high-explosive charge. Dummy: Ammunition with an inert warhead, inert primer, and no propellant; used for training or display. Blank: Ammunition with live primer, greatly reduced propellant charge (typically black powder), and no projectile; used for training, demonstration or ceremonial use. Field artillery system Because modern field artillery mostly uses indirect fire, the guns have to be part of a system that enables them to attack targets invisible to them, in accordance with the combined arms plan. The main functions in the field artillery system are: Communications Command: authority to allocate resources; Target acquisition: detect, identify and deduce the location of targets; Control: authority to decide which targets to attack and allot fire units to the attack; Computation of firing data – to deliver fire from a fire unit onto its target; Fire units: guns, launchers or mortars grouped together; Specialist services: produce data to support the production of accurate firing data; Logistic services: to provide combat supplies, particularly ammunition, and equipment support. All these calculations to produce a quadrant elevation (or range) and azimuth were done manually using instruments, tabulated, data of the moment, and approximations until battlefield computers started appearing in the 1960s and 1970s. While some early calculators copied the manual method (typically substituting polynomials for tabulated data), computers use a different approach. They simulate a shell's trajectory by 'flying' it in short steps and applying data about the conditions affecting the trajectory at each step. This simulation is repeated until it produces a quadrant elevation and azimuth that lands the shell within the required