Even a grinder alone won’t work, because the original stamping leaves readable marks in the underlying crystalline structure of the metal UNDER the grinding zome. So first you have to peen the crap out of the number with a punch, and THEN grind it off. Very few crooks no this, so guns are commonly traced even if they have ground off the visible numbers.
Fun facts to know and tell.
And if it WAS traced... what good does it do, usually. Probably was stolen. It’s the person who was caught wielding the weapon in crime who’s in big dutch here.