subcaliber

from The Collaborative International Dictionary of English v.0.48

Subcaliber \Sub*cal"i*ber\, a.
   Smaller than the caliber of a firearm. [Written also
   {subcalibre}.]
   [1913 Webster]

   {Subcaliber projectile}, a projectile having a smaller
      diameter than the caliber of the arm from which it is
      fired, and to which it is fitted by means of a sabot.
      --Knight.
      [1913 Webster]