Schematic for coil gun

[Deltroz 114]

Can someone explain how a coil gun works

i understand megnatism and I have a design in my head that I have been thinking about for 2 day strait but how would the gun know to shut of when the projectile is in the middle other than ir sensors or timers 

and would it be more efficient with or without capacitors

Could I make one out of old speaker voice coils?