There is no law requiring you to report firearm destruction or disposition to anyone.
Pardon me, if I've missed a California or New Jersey law.
No one's talking about observing any law in reporting a destroyed receiver, just CYA to avoid any future money spent on lawyers and court systems regarding the law regarding removal of a serial number.
I have a daughter. I tell her, "911 is what you dial after you're raped. 1911 is what you should have before they try."