Unless the weapon may have been stolen, lost, brought away or just moved in general, it shouldn’t be touched. If the weapon isn’t taken from the victim, it should be left where it was found. If the weapon gets taken from the attacker or the victim, the officer that takes it needs to tag it as evidence, and fill out all of its reports.
If a weapon has to be moved, the person that moves it needs to mark where it was at first. In all other cases, the scene just really shouldn’t be messed with.