the time to fire. An executive order to fire is given to all guns in the fire unit at the correct moment to fire. When each fire unit fires their rounds at their individual firing time all the opening rounds will reach the target area almost simultaneously. This is especially effective when combined with techniques that allow fires for effect to be made without preliminary adjusting fires. Multiple round simultaneous impact Multiple round simultaneous impact (MRSI) is a modern version of the earlier time on target concept. MRSI is when a single gun fires multiple shells so all arrive at the same target simultaneously. This is possible because there is more than one trajectory for a round to fly to any given target. Typically one is below 45 degrees from horizontal and the other is above it, and by using different sized propellant charges with each shell, it is possible to utilize more than two trajectories. Because the higher trajectories cause the shells to arc higher into the air, they take longer to reach the target. If shells are fired on higher trajectories for initial volleys (starting with the shell with the most propellant and working down) and later volleys are fired on the lower trajectories, with the correct timing the shells will all arrive at the same target simultaneously. This is useful because many more shells can land on the target with no warning. With traditional methods of firing, the target area may have time (however long it takes to reload and re-fire the guns) to take cover between volleys. However, guns capable of burst fire can deliver multiple rounds in a few seconds if they use the same firing data for each, and if guns in more than one location are firing on one target they can use Time on Target procedures so that all their shells arrive at the same time and target. MRSI has a few prerequisites. The first is guns with a high rate of fire. The second is the ability to use different sized propellant charges. Third is a fire control computer that has the ability to compute MRSI volleys and the capability to produce firing data, sent to each gun, and then presented to the gun commander in the correct order. The number of rounds that can be delivered in MRSI depends primarily on the range to the target and the rate of