Quote:"Out flew a red-golden dragon--not life-size, but terribly life-like: fire came from his jaws, his eyes glared down; there was a roar, and he whizzed three times over the heads of the crowd. They all ducked, and many fell flat on their faces. The dragon passed like an express train, turned a somersault, and burst over Bywater with a deafening explosion."
That quote, from The Fellowship of The Ring is very similar to what you're talking about, and it's been discussed by scholars of his work for a long while.  We doubt that hobbits, despite having access to both gunpowder and clockworks, obviously the most technologically advanced of the peoples of middle earth, knew what express trains were, but there it is in the text.
There are two methods of handling this kind of anachronism. Â You can either go with it, as Tolkien did in this passage (but no others), risking jarring the reader out of their immersion, or you can "localize" it. Â Either Orrin swung his spear like a baseball bat, or he did a heavy, full bodied horizontal swing.
The question here is the audience to whom you are writing. Â If you feel that you are writing to an audience that desires immersion, then you should localize. Â What bursts through a wall like an unstoppable tank might instead crash through the masonry like a raging behemoth. Â If your audience desires clarity over artifice, then more modern terminology might be more appropriate. Â If, for example, someone new to the lore heard the line about a behemoth, it would do them no good if they were unaware of what, exactly, one was.
