Spotting rifle A spotting rifle is a small - calibre rifle used as a sighting device for artillery . The ballistics of the spotting rifle are matched to those of the artillery piece , so that if a shot from the spotting rifle lands on the target , it may be assumed that the main weapon will also do so . Anti - tank recoilless rifles . Spotting rifles were most commonly used with recoilless rifle s as anti - tank artillery , from the 1950s to the 1970s . These weapons are used for direct fire with line - of - sight visibility to the target . As recoilless rifles have a relatively low muzzle velocity and thus a high trajectory , accurate ranging is important An optical sight can measure the bearing to a target , but not easily or accurately measure the range . Optical rangefinder s were too large for lightweight portable artillery , and the later development of laser rangefinders would in turn make spotting rifles obsolete . The British BAT series of 120 mm anti - tank recoilless rifles used a variety of sighting systems . The original BAT used a purely optical sight . The MoBAT , a lightweight and more portable version , had a 7 . 62mm calibre Bren light machine gun . The final version , the WOMBAT , used the same American Remington M8C . 50 in as the M40 recoilless rifle . Both of these were used as gas - operated semi - automatic rifles . The . 50 was not , as often assumed , a development of the . 50 M2 machine gun using the 12 . 7mm×99mm NATO cartridge , but was a shortened 12 . 7mm×76mm cartridge . Where a vehicle , such as the M50 Ontos , used multiple barrels , spotting rifles were usually provided for each barrel so that each could be accurately boresight ed to its own barrel . The Ontos used six barrels with four spotting rifles ; two without , to avoid making an already tall vehicle even higher . This provision may have been over - generous , although it did also increase the magazine capacity available for repeated ranging shots . Many photographs of the Ontos in service in Vietnam show it with only two of the spotting rifles installed in their mounts . The Ontos also often only carried four or even two main rifle tubes . Rocket launchers . Perhaps the smallest weapons to use spotting rifles are man - portable shoulder - launched rocket launcher s . The British 94mm LAW 80 is a rocket launcher developed as a replacement for the LAW 66 and Carl Gustav to give a more powerful infantry anti - tank capability . This uses a 9 mm rifle with a 5 round magazine and a particularly unusual design of ballistically matched cartridge . This same spotting rifle and round was later adopted for the US 83 mm Shoulder - launched Multipurpose Assault Weapon ( SMAW ) . Tank main guns . A rare few tanks have used spotting rifles , termed a " ranging gun " for ranging their main gun . This was a short - lived use , from the period in the 1950s when the increasingly powerful tank guns ( and their reduced capacity for spare ammunition ) encouraged a more carefully ranged first shot in the hope of an immediate kill , and the later development of laser rangefinders . The British Centurion Mk 5 / 2 with its new 105mm L7 main gun introduced a ranging gun . This was derived from the M2 Browning design , both the L6 variant of the gun and also its 12 . 7×99 mm ammunition . This ammunition was once again ballistically matched to the main gun . As this was a rather higher velocity round than the recoilless rifles , the case was the same as the M2 and the rounds could be used in either weapon . The enhanced tracer effect of the spotting round was sometimes used in an M2 . Mounted in the Centurion it was alongside another conventional co - axial machine gun , the ranging gun being the one in the mantlet closest to the main gun . The ranging gun was considered a success on the Centurion and was fitted to several marks of it , including retrofitting to earlier models with the 84 mm gun . The same ranging gun principle was later used for the 120mm gun of the Chieftain . It was considered less successful with the more powerful gun , as the range for accurate ranging was limited to 2 , 600 yards — beyond this the tracer element burned out and the fall of shot could no longer be seen . As the gun ' s own range with HESH could be up to 8 , 700 yards , this represented a serious limitation on performance . Battle planning at this time also saw a need for combat at increasing ranges , from the 2 , 000 yards expected for the Centurion ' s vintage out to 3 , 000 yards and beyond . As soon as they became available , the Chieftain rapidly adopted a laser rangefinder instead . A later UK light tank using a ranging gun is the FV101 Scorpion , which uses a 7 . 62mm L43A1 machine gun . This is mounted as a normal co - axial machine gun and may be used as such . it is a development of the standard British L8 Tk MG , used on a number of AFVs . It has an additional barrel bearing that is used to mount the gun . This bearing , rather than mounting by the receiver as for the L8 , gives a more consistent point of impact between a cold and hot barrel , which is important for use when ranging .