A mass accelerator propels a solid metal slug using precisely-controlled electromagnetic attraction and repulsion. The slug is designed to squash or shatter on impact, increasing the energy it transfers to the target. If this were not the case, it would simply punch a hole right through, doing minimal damage.
Accelerator design was revolutionized by element zero. A slug lightened by a mass effect field can be accelerated to greater speeds, permitting projectile velocities that were previously unobtainable. If accelerated to a high enough velocity, a simple paint chip can impact with the same destructive force as a nuclear weapon.
However, mass accelerators produce recoil equal to their impact energy. This is mitigated somewhat by the mass effect fields that rounds are suspended within, but weapon recoil is still the prime limiting factor on slug velocity.