Ok so first of all not even Vin or Elend ever burned metal outside their body (to my knowledge) and they were one of the best trained mistborn/allomancer who probably ever lived, and someone who was a first gen mistborn who would have allomancy that would be, at base, much more powerful than that of any other allomancer/mitsing/mistborn. It may be possible but i highly doubt that there is anyone alive in era 2 mistborn who can. (after reading the post currently above this i see pre-Kelsier Vin brought up as potentially burning things outside of her stomach prior and I dont think that was the case as most water has trace minerals and it also states in book that much of the utensils and cups used to eat were made of either copper or pewter but im not sure which and i dont have the book on hand. So it is much more likely that she was burning those traces when they reached her stomach.)
Second of all, you mention that Ranette is trying to develop ways to kill mistings easier while being a misting herself. May I remind you that many of the opponents faced in the first series were mistings and sometimes (but more rarely) were even full fledged mistborn! If they had a weapon that would allow them to reach their goal of defeating their opponent, even if that meant there was a weapon that would make them easier to kill in turn, they would probably want that weapon. Also, this is a rather poorly though out point as many of the criminals we see Wax and Wayne track are mistings or twinborn, so it would be almost necessary to have weapons that can make taking out a potentially hostile misting out.