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]