crouching in trenches – 1/25–1/100 Airburst munitions significantly increase the relative risk for lying men, etc. Historically most casualties occur in the first 10–15 seconds of fire, i.e. the time needed to react and improve protective posture, however, this is less relevant if airburst is used. There are several ways of making best use of this brief window of maximum vulnerability: ordering the guns to fire together, either by executive order or by a "fire at" time. The disadvantage is that if the fire is concentrated from many dispersed fire units then there will be different times of flight and the first rounds will be spread in time. To some extent a large concentration offsets the problem because it may mean that only one round is required from each gun and most of these could arrive in the 15 second window. burst fire, a rate of fire to deliver three rounds from each gun within 10 or 15 seconds, this reduces the number of guns and hence fire units needed, which means they may be less dispersed and have less variation in their times of flight. Smaller caliber guns, such as 105 mm, have always been able to deliver three rounds in 15 seconds, larger calibers firing fixed rounds could also do it but it wasn't until the 1970s that a multi-charge 155 mm howitzer, FH-70 first gained the capability. multiple round simultaneous impact (MRSI), where a single weapon or multiple individual weapons fire multiple rounds at differing trajectories so that all rounds arrive on target at the same time. time on target'', fire units fire at the time less their time of flight, this works well with prearranged scheduled fire but is less satisfactory for opportunity targets because it means delaying the delivery of fire by selecting a 'safe' time that all or most fire units can achieve. It can be used with both the previous two methods. Counter-battery fire Modern counter-battery fire developed in World War I, with the objective of defeating the enemy's artillery. Typically such fire was used to suppress enemy batteries when they were or were about to interfere with the activities of friendly forces (such as to prevent enemy defensive artillery fire against an impending attack) or to systematically destroy enemy guns. In World War I the latter required air observation. The first indirect counter-battery fire was in May 1900 by an observer in