subcaliber

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Smaller in caliber than the barrel of the gun from which it was fired. Used of projectiles.
• adjective Relating to or firing such projectiles.

from The Century Dictionary.

• Of less caliber: said of a projectile as compared with the bore of the gun. See subcaliber projectile, under projectile.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Smaller than the caliber of a firearm.
• adjective 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Smaller than the caliber of a firearm.