It depends on how it was “safed”.
I’ve seen lots of bazooka and recoiless rifles made safe by cutting a big hole in the side of the tube where your face normally is when firing.
Cannons are often made safe by filling or plugging the barrel with cement.
I honestly don’t know how they’d make things like non-nfa machine guns non functional, but I expect it’s often something where they remove the bolt altogether, or remove the hammers or striker etc.
Some parts are easy to replace. Others are very difficult. But the ATF usually requires something irreversibly damaging be done. It depends on exactly what the weapon is and how it works.
