A mass accelerator is a weapon that uses mass effect technology to launch projectiles.
Mass accelerators are common weapons in the Mass Effect setting. Because mass effect technology scales easily, mass accelerators range in size from handguns to starship weapons that run the length of the ship.
Mass accelerator small arms function by removing a small piece of metal from an ammunition block, shaping it into a projectile, and propelling it with a mass effect field. Larger weapons use similar principles, all the way up to dreadnought mass accelerators that fire 20kg metal slugs at over 4000 kilometers per second, so they hit like 38 KT bombs.
The realism of mass accelerator weapons is dubious. According to the codex, a mass effect field reduces the mass of the projectile, allowing it to be accelerated faster. After the projectile leaves the weapon, its mass returns to normal without its velocity decreasing, which appears to violate conservation of energy. Presumably the weapon's power supply provides all of the kinetic energy of the projectile, and this method just makes the process more efficient or circumvents a limit of the technology used to provide the accelerating force.
Mass accelerator weapons are known to conserve momentum, as recoil is a significant factor limiting the firepower of such weapons.