This is a Glitch I noticed a lot in Crucible.
People using swords without Heavy ammo
I cant seem to find any other post of this sadly. Also if you see them with a sword but you cant see the weapon report them because this glitch makes the Sword invisible. (Hence the name)
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As others have said, not a glitch. It works the same as regular melee attacks. I take it you haven't got a sword at all to make a post like this