Ok...
Long ago and far away, a clever man discovered that if you take a coil of copper wire, shove a magnet in the middle, and spin the magnet, it makes electricity...
Electric Dynamo
Later it was discovered that if you take a coil of copper wire, shove a magnet in the middle, and turn on the electricity, the magnet spins...
Electric Engine
Later another man discovered that if you open out the outside of the engine flat, you can make the magnet move along the 'electromagnetic track'...
Magnetic Linear Induction
It was suggested that this last version could be used to make frictionless (at least with the 'rail' because it floats just above it on a magnetic field) and virtually noiseless light railway systems for inner cities...
Mag-Lev
It was then suggested that this thing could be used to launch chunks of metal mined from the asteroid belt or the surface of the moon, back to earth, using solar or fusion power, as a 'cheap' way of replacing chemical fueled cargo rockets...
Mass Driver
Then true to form, the human race said "hey we can turn this into a weapon", and set about trying...
Rail Gun / Gause Gun / Mass Driver
There is no friction between the projectile, and the barrel in a maglev weapon, as the projectile sits in the middle of a magnetic field and does not touch the sides. This suggests high velocity. If you dont believe in this, smash your hard drive open. Modern hard drives use 'frictionless bearings' made with 'rare earth magnets', higher rpm for less power.
Freelancer already features a 'mass driver' like system, the common TradeLane. you enter the first 'hoop' and get shunted at high speed to the next 'hoop' and the next till you emerge at the other end, with the final 'hoop' reversing the process to turn your momentum back into energy, this is called 'magnetic braking', and a version f it is used on some modern subway cars, the engines are also dynamos, electricity is put in to accelerate the train out of one station, then as it aproaches the next station, the process is reversed, so the movment of the car turns the wheels, which spins the engine core slowing the car down and getting back quite a lot of the electrical energy put in during acceleration.
Oh, and 'Gauss' is a scientific unit of measurement named after a dead physicist, specialising in electromagnetism...
'Rail' comes from the original suggested application and first practical experiments moving a 'car' along a rail...
None of these weapons uses 'bore reduction' or chemical propellants of any kind.
'Mass Driver' comes from scifi authors back in the 40's, 50's and 60's, taking about using this tech for moving large chunks of mass through space cheaply.