The bow and the lance were the two usual weapons of soldiers in those countries, Jeremiah 6:23. The Persians were a cruel, bloody people. These phrases signify no more than that the enemies should come upon Babylon in a terrible manner, and prepared to destroy them.
Related Commentaries of Jeremiah 50:42
Jeremiah 50:42
42 They shall hold the bow and the lance: they are cruel, and will not shew mercy: their voice shall roar like the sea, and they shall ride upon horses, every one put in array, like a man to the battle, against thee, O daughter of Babylon.