Any gun is ok as long as you are the only one using it in that match. As soon as a second player on your team uses the same gun, it becomes OP. Additionally, you cannot kill a player more than once with the same gun. As soon as a player dies more than once in a match due to the same gun, that gun is now OP. Edit: I exclusively use slow-charge fusions in the Crucible and do fairly well. Still waiting on hate mail though.