In this paper, it describes a proposed cannon using a linear track with a series of large superconducting magnets.
The cannon’s advantages include near 100% efficiency on the energy transfer, minimal wear when each shot is fired, and the example designs have muzzle energies measured in gigajoules.
The main disadvantage seems to be that it is extremely long and heavy, versus a railgun, and is far too large to mount as a weapon on any military platform yet built.
Each magnetic is energized to just below the capacity limits for the material it is made from. Projectile gets attracted to the line of magnets, and when it passes through each one the magnet is “quenched” instantly, open-circuiting the magnet.
How would you do this, physically? You cannot use power control semiconductors in a superconducting ring circuit. I think a mechanical switch would be far too slow and it would arc.