The mishnah in Zevachim 73 brings the following case: A kli sharet made of copper is used to cook a sacrifice. The kli needs to be scoured and rinsed to remove traces of the sacrifice to avoid the problem of nosar. Before the kli can be cleaned, it leaves the Temple courtyard and while outside becomes tamei. The vessel still needs to be scoured and rinsed but because it's tamei it can't be brought back to the Temple and it's only the Temple that it can be cleansed.
The Mishnah's solution is for someone to punch a hole in the utensil so that it loses its designation as a kli. Once that happens, it's no longer tamei and can be brought back to the Temple where it can be scoured.
However, I have a question: while the Mishnah's solution obviously works, it is my understanding that metal utensils can be made tahor by immersion in a mikveh. Why does the Mishnah not offer that as a solution in addition to punching a hole in it?
Is there are special rule about utensils that are kli shares? Or is the answer because the Mishnah first talks about doing the same for an earthenware utensil (which cannot be purified by immersion) so it simply brings the parallel case for a copper utensil?
Anyone know the answer?