The shrapnel is the only projectile that may be fired from either 155-mm guns or howitzers, however, its use is limited to the old M1917-17A1-18M1 Guns and M1917-17A1-18 Howitzers for impact practice fire. Only in cases of extreme emergency would shrapnel be used for combat.
No information about functioning.
TM 9-1904, Ammunition Inspection Guide (1944)