The mobile version of the Long Tom rifle is slow, bulky and poorly defended, but it remains one of the most powerful long-range weapon systems still surviving from the days of the Star League. Though it lacks speed and grace, it can provide impressive fire support and defense. The Armstrong mobile Long Tom rifle variant is the last remnant of the Star League's main guardian defense system. This powerful and accurate long-range weapon can reduce most small 'Mechs to scrap heaps.
         The mobile Long Tom is actually a series of tracked vehicles working together to move and position the rifle for firing. However, the system can only be transported slowly. Garrisoned worlds in and near key cities usually possess this system.

         The history of the mobile Long Tom is filled with stories of how the system saved entire cities with its awesome firepower. This immense weapon has enough energy to level an oncoming BattleMech.
         The mobile system operates as a train of five tracked transports. The lead transport is the piloting cab as well as the targeting center, with the targeting system placed on top of the cab. This massive gun cannot be fired while being transported. With its twelve tons of armor and twenty tons of ammunition, the vehicle travels at a snail's pace, topping off at 25.6 kilometers per hour and needing frequent refueling. The sheer weight of the unit requires special heavy roads or rail tracks on which to move. A mobile Long Tom rarely travels cross-country, as there is a good chance that it will get stuck. Fifteen crew members are required to fire and drive the Long Tom.
         The Long Tom excels at long-range attacks but is relatively weak at close quarters, and so tanks, infantry and BattleMechs often defend the unit. Though the Long Tom carries four side-mounted machine guns for close defense, these are hardly enough to take on an armed 'Mech. However, the greatest threat to the mobile Long Tom are aerospace fighters. Rifleman 'Mechs, known for air strike support, are frequently stationed with this unit.

         The best known variant of the mobile Long Tom system is the rail version, though it is even more rare than the tracked system. The rail version does not have the tracked version's freedom of movement, but it can travel faster via rail. Most of the Inner Sphere's rail tracks are in poor shape, however. This variant has a crew of twenty-five and is virtually unarmored. It is officially listed as the LT-MOB-50 in Star League records.

         In 2888, House Kurita launched yet another raid on the Davion world of Errai. On that lonely world was the city of Trent, a well-known storehouse for Davion troops and 'Mechs. Two Long Tom mobile systems used Trent's massive city square as a firing platform while the local 'Mech forces fought to hold back the Kurita raiders. During the battle's initial stages, the Long Toms destroyed more 'Mechs than all the Davion ground forces combined. Eventually, the Kurita raiders withdrew. Before they left, however, an aerospace fighter wing managed to cripple both Long Tom defenders. The Long Toms destroyed more than ten fighters before succumbing to the Kurita barrage.
         While Houses Marik and Liao were fighting for the city of Garth on the planet Berenson, the Long Tom once again proved that it could play both a defensive and an offensive role. The Marik attackers set themselves up in the city of Tromoth. From there, they moved up a Long Tom rifle system to bombard the city of Garth. At the same time, House Liao moved up one of their Long Tom mobile systems to attack the Marik raiders. The two guns attempted to destroy each other for nearly a week. Finally, spies in Garth sabotaged and disabled the Liao gun, giving the Marik weapon free rein to bombard the rest of the city. This gave the Marik attackers the break they needed to take Garth.
         In 3002, House Marik raided the city of Treth on the planet Loric in the Lyran Commonwealth. By Moving and firing their only mobile Long Tom, the Steiner defenders fooled the Marik forces into thinking they faced three Long Toms in and around the city. The Free Worlds League troops withdrew quickly without seriously damaging the Steiner forces.
         The mobile Long Tom is an uncommon weapon, and troopers consider it an honor to serve on this cannon's crew. Very few are stripped for parts; most nonfunctioning Long Toms are maintained as is in case the parts needed to repair them are found. If the mobile system is destroyed, the weapon is mounted in a static defense area such as a fortress or city.