The M51 Infantry Fighting Vehicle was a wheeled infantry fighting vehicle designed by the United States Military starting in 2070, after experience in the American Annexation of Canada proved that the venerable M113 Armored Personnel Carrier, even with its latest upgrades, was woefully outgunned by the Canadians MC12 Stryker infantry fighting vehicles. In 2068, the US Department of Defense issued specifications for a wheeled fighting vehicle with a cold fusion engine capable of traveling at speeds of at least 140 miles per hour, with armaments capable of engaging infantry and, if necessary, tanks.
The final design chosen was a vehicle heavily influenced by the MC12, with similar titanium-depleted uranium composite armor and an engine powered by a cold fusion reactor. On a paved road, the vehicle could reach speeds of 150 miles per hour, with a top speed of 60-70 mph off road, however, as it often operated in conjunction with tanks, the M51 was often forced to travel at much slower speeds to keep up. The vehicle was armed with 120mm low-recoil, short-barreled gun-launcher, which relied on anti-tank guided missiles or plasma shaped charge shells to engage enemy armor, while high explosive, canister, and "mini-nuke" rounds were available for engaging infantry and structures. The gun was mounted in a one-man turret and fed using an autoloader, with a standard capacity of 30 rounds and five ATGMs. The turret also mounted a co-axial 7.62mm machine gun and turret-top pintle mount for an M2 .50 caliber machine gun, minigun, or Gatling laser. Finally, the tank carried an M-78 Tactical Grenade Launching System previously mounted on the M56 Main Battle Tank. The M78 consisted of two twin-linked automatic 40mm grenade launchers on the front of the vehicle with a vertical elevation of up to 65 degrees, a depression of -10 degrees, and left to right traverse of 35 degrees. These weapons had a range of up 400 meters, were often loaded with a combination of airburst and impact detonated rounds, which proved high effective against infantry and unfortified buildings in the Sino-American War. The M78 is operated by a fourth (in addition the commander, driver, and main gunner) crewman who also acts as radio operator and co-driver.
- M51AA Mobile Anti-Air Vehicle: Light self-propelled anti-aircraft gun and missile system, armed a turret modified to carry a 20mm Vulcan rotary cannon or heavy Gatling laser and two FGM-219 "Quadbarrel" quad-shot dual purpose missile launchers.
- M51C3 Command, Control, and Communications Vehicles: Internal troop bay replaced with radio and computer suite for use as a forward command vehicle. Armament identical to standard model, identified by retractable radio antenna on top of turret.
- M51EW Energy Weapons Carrier: Standard Turret replaced with turret armed with high-energy laser weapon designed for penetrating armor.
- M51M Mortar Carrier: Troop bay replaced with 120mm mortar and ammunition stowage, with firing port on back roof of vehicle. Turret and other features identical to standard vehicle.
- M51MGS Mobile Gun System: Fire support variant with turret replaced with 120mm high velocity gun in unmanned turret equipped with autoloader. Troop bay divided between fire control station and additional ammunition stowage.
- M51ARRV Armored Repair and Recovery Vehicle: Armored repair and recovery vehicle with turret replaced with crane for towing armored vehicles. Folding stabilizers located at front and rear on the sides of the hull. Troop bay modified for stowage of equipment. Pintle-mounted M2 .50 caliber machine gun next to commander's hatch for defensive armament.
- M51E Engineering Vehicle: Armored engineering vehicle with mount for dozer blade, mine plow, or mine roller. Turret replaced with mount for either backhoe bucket, crane, or mine-clearing line charge (MCLC), folding stabilizers located on sides of the hull at front and rear. Troop bay modified for stowage of equipment. Pintle-mounted M2 .50 caliber machine gun next to commander's hatch for defensive armament.
- New California Republic Military (~60 in inventory)
- US Military Remnants
- Brotherhood of Steel
- Commonwealth Minutemen (limited numbers post-2288)
- Caesar's Legion (Limited numbers, all destroyed at Battle of Dolan Springs and Siege of Nova Roma)
- Union of the Ute and Navajo Nations (Ten vehicles taken from pre-war New Mexico National Guard. Six recovered in working order after liberation from Legion).
- The Armored Merchant (California and Southwest region caravan operating out of a recovered and repaired M51)