The activated ability alone pushes this past common or even uncommon.

Compare your card with e. g. Heavy Arbalest, Throwing Knife, Wolfhunter's Quiver.

This card gives you a Ninja's Kunai-plus every turn in addition to +2/+0 snd reach (which are irrelevant most of the time because you get to cast Flame Lash every turn - representing a 5-turn clock that can't be blocked and stays on board past creature removal, and is removal for mid-sized creatures as well.

This card isn't just "to good for common", it's "too much for a card". Might not need Aladdin's Ring costs, but anything repeatedly putting out 4 damage to an target is rare+ even after balancing.

I haven't even talked about it before, but now that the equip cost is zero: Equip costs can be paid multiple times a turn to move around an Equipment. If you control this and five untapped creatures, you can reduce a player's life total by 20 life without any further investment - and if you survive another turn cycle, you can do so again. And I'm not even counting the removal option which will really help with that.

This design is in no way a common, but the equip cost just pushes it further. It's not even reasonable as a mythic.

Why does a four-mana Equipment allow you to repeatedly (not to mention again multiple times a turn) do as much damage as a four-mana spell that's used up after one use?

Look up the above-mentioned Heavy Arbalest, and you'll see in just how many ways this card upstages it.