The correct use of the term refers to pieces of the object STRUCK, flying off, not the pieces of the striking object.
This first became widely known from WW I tanks, and still is an issue, but not to the degree it was then, or up through early WWII. The worst offenders were tanks with brittle hard armor plate, and those with riveted construction.
Especially the rivets. Rounds striking the rivet heads on the outside of the tank frequently broke the rivets, sending high speed metal fragments around inside the tank. Very bad for the crew.
Like every other technical term in English, there is a correct definition, and then there is the way people use it incorrectly in common speech.
Many people refer to shell fragments and spall as "shrapnel" but while the common use today, tis technically incorrect. They are fragments, not "shrapnel". But, like Kleenex and Band-Aids it has become the generic term used.