Both words, i.e., fling and hurtle, are (or have been) used correctly in your above example sentence.
Some things to consider here: Since the verb to fling (and, hence, its past tense, flung) carries the meaning "[t]o throw with violence," then the construction "to fling ... hard" may be seen by some as a redundancy. Why not then instead? :
He threw the ball. It came hurtling toward us.As I've already stated, the use of flung would not be incorrect here. So why do I suggest the use of threw here, instead of flung? Further, why do I suggest two sentences instead of one joined by the conjunction and? :
(1) The verb to throw is more direct here; though one may, of course, fling a ball, the more usual verb for a ball is to throw; (2) The word flung, being a "twice irregular" past tense English verb (the usual irregular conjugation: present, past, past participle: ring, rang, rung; sing, sang, sung. But fling, flang, flung??? No! - to fling is "twice irregular," fling, flung, flung), seems to draw unnecessay attention to itself here; (3) The ensuing word, hurtling, describes the force of the thrown ball; thus, a similar kind of redundancy in the construction "flung ... hard" is present here with "... flung ... came hurtling." Let's eliminate redundancy altogether; (4) Now, with the more usual verb form, threw, the reader needs the second sentence (in your case the second clause) to complete the idea of the force by which the ball has been thrown; (5) Changing your one sentence into two by getting rid of the conjunction, and, forces the reader to at first separate the two actions; (6) With the first sentence now flowing immediately into the second (without the and), the reader is forced to make the connection about the speed of the thrown ball; (7) The element of surprise for the reader is now highlighted, reflecting the same kind of surprise for those in the second sentence who are experiencing the hurtling ball.
You may say either "... are correctly used ..." or "... have been correctly used...". The slight difference in meaning is one that occurs between present and present perfect "tense."
Student or Learner