another thing to think about... were there finger prints on the gun ??? even if they were pulled, does a finger printing kit actually remove the finger prints, if not, maybe wiping down & oiling up a gun, may not be an option on any case that went to trial???
I'd think it in best interest to keep all evidence in as collected shape... if you went to trial, & were asked if a particular gun was yours, & it was all rusty, or if the gun had rusted beyond function, & in a murder trial, the gun goes into court in a "non firing" condition, a sharp lawyer may get someone off because the gun couldn't have killed someone, if it were rusted shut...
I'm good friends with several LEO's, & while the guns are tools, very few cops are enthusiasts, so one who is not an enthusiast reading this thread is pretty unlikely... enthusiasts could understand the value of a particular weapon... cops who are not enthusiasts are not likely to care about any collectability of some old steel gun, that got all crusty, or got extra numbers engraved in it... if the gun were a new M&P or Glock, it wouldn't have rusted like that old piece of crab... 1911, & as a tool, extra numbers don't effect the usefulness or effectiveness of their tools
In life you either make dust or eat dust...