Legal Question in Criminal Law in California

For your sake I hope they don't find the body in the pond ;)

Seriously, while I have no idea how it is you are being charged with carrying a firearm that was at the bottom of a pond, the best way to present your defenses to the charge is by retaining an attorney.

You can ask for anything you like, by means of a proper motion to the court for an order to compel. If you don't know how to do these things effectively, then hire an attorney that does, and who can represent you against a professional prosecutor intending to convict and jail you.. If serious about hiring counsel to help in this, and if this is in SoCal courts, feel free to contact me.

I'm trying to follow your question - I'll assume that you're being charged and part of your defense is that this isn't the right gun or that you didn't possess it.

You want to know if you can compel the police to process the gun, right? In theory, yes. You and your lawyer should be discussing this in greater detail - is it something you really want to do? Or will the lack of evidence play out better for you? Would it be better/easier/etc to rely on the failure to rule out other possibilities?

That's something for you and your lawyer, not our guesses here.

I think you watch too much television.