Faux-Bow, no doubts. Hit hard with an RSCB, no strings, and a bad seal. Pretty sure it can take a hopper and pop through paper even with a Lowes FBR slug. Only homemades beat a Faux-Bow. Take my advice on that.
That uses passive magnetic levitation, not propulsion. You don't need or want magnetic levitation because your plunger is being guided by its seal against the inside wall of the plunger tube.
If you want linear motion you need a linear drive, and you can't get that with just magnets. Linear electric motors require a moving electromagnetic field along a sequence of coils.
Unnamed Cell gave me some advice on this. Coils would be around the outside.
Another question is how would I then secure the catch, and other stuff using bolts/screws, to the blaster?