Indeed. It's not difficult to make a gun in a home workshop. I'd guess the main challenge would be finding the right grade of steel for making the chamber, barrel and firing pin. Back in the 1980s, my dad made a working model of an anti-aircraft gun that used a cut-down rifle barrel and which could (and did) fire .22 rounds. The plans were published in the Model Engineer around 1942.
Weld can be bored through or ground off. I can't help thinking that the safest way to deactivate a firearm would be to so weaken the chamber that it would explode if the gun was fired.