This is more of an English usage question than Greek, but even modern translations are split on this. In Eph. 6:16 should τὰ βέλη be darts or arrows? Development of weaponry (using modern English) was the spear, the atlatl using darts, the bow using arrows, and the crossbow using darts. The atlatl became obsolete with the bow, and the crossbow had not yet been invented in the first century. In fact, the crossbow made armor obsolete because it had the power to penetrate the armor at that time and nail victims in their armor. Older English may have called arrows darts, but this seems misleading in Modern English. Because of what the word can mean (see LXX usage below), to resolve this probably takes knowledge of Roman warfare, i.e. what they used to cast fire.
Appendix: Since the lemma βέλος only occurs here in the New Testament, here is how it is used in the LXX and the Hebrew words it translates (generated with Logos Bible Software):