Source : Webster's Revised Unabridged Dictionary (1913)

Compression projectile \Com*pres"sion pro*jec"tile\
   A projectile constructed so as to take the grooves of a rifle
   by means of a soft copper band firmly attached near its base
   or, formerly, by means of an envelope of soft metal. In small
   arms the modern projectile, having a soft core and harder
   jacket, is subjected to compression throughout the entire
   cylindrical part.