So you have no explanation whatsoever for why the DPD would switch the pistols, frame a person they knew was innocent, and let the guilty person who they obtained the pistol from go free? It just could have happened. Got it.
Why should I need to explain that? It's another strawman.
The sole purpose of a chain of custody is to establish that a piece of evidence is in fact related to the alleged crime, rather than having, for example, been "planted" fraudulently to make someone appear guilty. In other words, to limit the possibility that a claim of "it could have happened" is successful and thus questioning (at the least) or destroying (at the worst) the evidentiary value of that piece of evidence.
It's a law enforcement officer's job to ensure that a credible, well documented chain of custody is established for each piece of evidence. You don't get to say "it doesn't matter that there is no credible chain of custody, because you can't explain why the police would tamper with the evidence". That's not how it works.... You don't know that?
Oh wait, I'm asking a die hard LN who will use any excuse to keep Oswald in play as the lone gun man. So, who cares about a chain of custody, right?