If we can only accelerate mass to relativistic speed by removing the effective mass. To get a Atsterorid up to C it would have to be effectively masless. To make a warp speed projectile would have to involve some crazy math so the tidal forces don't just shear it appart. Like the hyperfast low mass atoms hitting the decelerating atoms at the front of the object.
In enders game the AI would feather the FTL drives so that ships effectively stopped instantly as mass returned regular physics decelerated the object down to speeds available to regular physics. Which is a little handwavy but not actually that bad