You're not in trouble in a lot of US states, just the FEW that are commie run progressive utopias: NY, NJ, CA, and a couple more.
If you find a gun in the US it's simple. A few of my friends have been lucky fishing and hunting. Step 1: Contact police with serial number to make sure it's not stolen. If not stolen, proceed to step 2. If stolen, turn over to police to give back to rightful owner. Step 2: Generally after 30 days if no one comes foward to claim their property, it becomes yours. It's just like finding money, cuff links, or a guitar.
If you are faced with an intruder wouldn't you grab the best tool available to preserve your life? I'm sure he wasn't thinking, "Oh my, my shotgun certificate isn't valid and I found this gun without getting permission from my enslavers. Therefore I deserve to die/be victimized."
Your thought process is why crime is so high in the UK and the criminals have MORE rights and consideration than the victims. I can hear it now, gun crime in the US is......Yada yada yada, I mean crime regardless of the means used. A rape or robbery with a knife or physical force is the same as a rape or robbery committed with a belt fed machine gun.